I know little about this and I am learning guitar and really enjoy passing time improvising off of scales. I also have an acoustic bass that I enjoy as well.
If two people are jamming - one bass and one guitar - does the bass player keep the time? What if there are two guitars?
Thanks
In most styles of music I am familiar with, one player will generally be mostly responsible for playing rhythm - keeping the time - at any one time. Most often it is the bass. In rock, bass and drums are generally considered the rhythm section. But you could always trade off between bass and guitar - or 2 guitars.
Of course, both players should be aware of and playing in rhythm. If a guitar takes a break/lead - that lead generally should take place within the rhythmic structure of the song. I play upright in several bluegrass/oldtime jams. One of the most thrilling things for me is when a good picker takes off on a lead, and I have no idea where they are going with it, but when the pattern comes back around I know they are gonna nail that downbeat. Really sucks when someone thinks they are taking a break and, instead, they are just aimlessly noodling.
I guess in an unusual jam, if the bassist were more accomplished they might play more “lead” material and the guitar might play rhythm. But that would be unusual. In a jam with multiple guitars (and other instruments), most often a couple of the guitars will focus on holding down the beat.
In some styles of jazz - which I am ignorant about - it appears that no one player strictly plays rhythm. Instead, all can be improvising - yet keeping track of the underlying rhythm amonst themselves.
I’ve picked with hundreds of folk. I STRONGLY suggest you get a metronome and practice playing with it. I’ve been playing bass for sometime and take some pride in being awfully steady - yet I still put in regular metronome time. It is VERY common for someone to join a jam after noodling away by themself in their family room. They have NO IDEA what it means to play in time and with other people. They sit there with a big old smile on their face, but they are killing the jam. I’m fortunate that the folk I play with in my string band and classical ensemble have no objection to pulling out the metronome when needed. Being solid will go a long way in making a picker a valuable addition than the ability to play fancy chords or a lot of fast notes.
Second the recommendation for practicing with a metronome.
There’s nothing more frustrating than trying to jam with a player who doesn’t have at least basic time sense.
One suggestion: to make it feel a bit more like a jam and less like a technical exercise, use a drum machine rather than a classical metronome. Even a quite cheap one will have a variety of patterns that you can choose from.
It’s a good idea to record yourself playing along with your rhythm track.
While you’re playing, you are concentrating on the actual playing, but listening back, you can judge how well you are staying with the rhythm.
This can be a bit intimidating at first: I think of myself as a good solid groove bass player, but on playback I can hear the imperfections, which encourages me to pay more attention to staying “in the pocket”. 
This I’m aware of as I listen to, and watch, lots of metal, hard rock, and stoner rock. However, I’m just a middling guy with an Epiphone Tony Iommi signature SG (hey I can try, right 
?), as well as an acoustic six-string, and the acoustic bass I mentioned in my OP.
I’ve never jammed with anyone so I was curious about how this would work.
and
The metronome thing is actually the genesis of my question; I’ve noticed that even if I’m listening to a backing track or metronome I can still find myself sort of “drifting” off the rhythm. So last night, before my post here, I spent about half an hour doing scales and improvising, while focusing on my metronome.
Thanks @Dinsdale and @xtenkfarpl for your insights.
Use the metronome both as the 1/3 and the 2/4. Keep at it. If you are drifting - stop drifting. Slow the metronome down until you can stay solid.
@Dinsdale, thanks for that. I just retired a year ago and I am trying to take up a more methodological approach to get a better grounding on some of the fundamentals and, from what I’ve read (elsewhere and above), this is extremely important.
missed the edit window:
And @Dinsdale and @xtenkfarpl, do either of you find it challenging to stay on rhythm?
These days, not really. But I’ve been playing for decades, and have been fortunate enough to play with one or two really good drummers. Which is vital experience for a bass player who wants to be part of a good rhythm section.
As Dinsdale says: if you are having trouble, slow the beat down.
Get it solid first, don’t struggle and “practice sloppy”.
It’s been a long time since I used a hardware drum machine: these days any practicing I do is in my home studio, where backing tracks can be generated from my audio software.
But a quick web search shows that there are some free drum machine apps for smartphones.
So you don’t even need to buy anything…
I have a ZOOM G1 Four (https://images.app.goo.gl/58Vvk) which has a rhythm function with a variety of settings. I also have a metronome so, for me, I just have to start slowly and focused, as mentioned by you folks.
No. Like xten said, if you want to be a decent bassist, if you want gigs, and if you want folk to want to play with you, the MOST IMPORTANT THING - arguably the ONLY thing - is to be solid. There are plenty of other folk who are eager to show off. But if the rhythm isn’t solid, the music will suffer.
Being in rhythm is more important than being in tune. It is preferable to play a wrong note at the right time than the right note at the wrong time. And be confident in the rhythm you are driving. There are few things worse in a jam than a tentative bass. At best, the other pickers will just ignore you and act as tho they don’t have a bass.
If you have a drummer, you lock in with the drummer and that’s it. And that’s enough. Your rhythm is what is going to get people tapping their feet, or up on the dance floor.
I play bluegrass and oldtime, where there generally is no drummer. So I am the rhythm section. Best case scenario is to lock in with a mando or guitar who hits the 2-4 backbeat. But lacking that, ANYTHING else I could bring is useless if I’m not driving a solid beat. So that is what I work on - and pride myself on.
Some folk I play with mention Strum Machine - a reasonably priced rhythm app which will play guitar parts for a tremendous number of songs. I don’t use it, but I’ve never heard a bad word about it. I think they might offer a free trial, so you mght want to check it out.
Final thought - no matter how solid you are, you can’t FORCE anyone else to keep a steady rhythm. At one jam, I regularly lock in with one particular guitarist, and we are as rock solid as you could hope for. But folk join the circle and either have no idea of what it means to play in rhythm - or they just don’t care. I used to think I could enforce a rhythm over the group by playing harder and harder. And other folk would say, “Listen to the bass!” But I learned that such efforts were fruitless. So now I show up early when the jam is small, and I just leave when the circle gets too large and the rhythm goes to shit. My friends have often told me how the jam went completely off the rails after I left, but that’s too bad. The group refuses to cap the number of pickers or do anything else to address the folk who detract more than they add. Making lousy music is not fun for me, so the only option for me is to absent myself.