Do any bluegrass groups ever use anything other than
Guitar
bass
fiddle
banjo
mandolin
(all acoustic only)
I guess if they show up with drums or a piano that’s a big mistake?
Some bluegrass bands have a dobro player.
Edit: Jerry Douglas is one of the best ever; he’s an incredibly talented musician.
Not accepted by purists but electric bass is sometimes seen. Carrying that acoustic bass can be a pain.
Autoharp and dulcimer are commonly found in bluegrass groups. In the early days, cello and viola were sometimes also found.
My jam group wouldn’t kick you out for playing an acoustic piano, if it were available, nor would we strenuously object to most acoustic string instruments. Heck, sometimes I even show up with an accordion. 
I’ve not seen a bluegrass band, playing as such, with anything but these on stage.
Some canonical bluegrass recordings do have drums, piano, or both.
Beyond bluegrass, in the realms of old-time country, mountain string, and “back-porch” jam groups–any of which might be taken for bluegrass by city folk–I’ve seen dulcimers hammered and lap, cello, autoharp, jaw harp, harmonica, penny whistle, tambourine, jug, saw, and uilleann pipes. And of course stomping shoes and jingle taps.
When I saw the Earl Scruggs Revue, it was banjo, guitar (acoustic), fiddle, electric bass, and a drum kit. But I’d say your basic bluegrass band is banjo, guitar, fiddle, bass. Dobro and mandolins are nice additions. Sometimes there are more than one guitar. And often a fiddle player will also play mandolin. They’re tuned the same.
Pedal steel guitar and keyboards show up around here when the Nashville & Memphis cats come a-callin’ in the summer. Drums, always. Both solid body and acoustic-electric guitars are used.
The quarter circle stompmight be considered a percussion instrument in it’s own right.
I think this starts getting into definition of a ‘bluegrass group’. For instance, when Bela Fleck was in New Grass Revival they played a lot of bluegrass, but also reggae, rock, etc. But since they were standard banjo, guitar, bass, fiddle, they’re considered a bluegrass group.
Bela Fleck and the Flecktones play a lot of bluegrass, but also jazz, rock, etc. But since they’re banjo, keyboards, and, um (looks it up) “drumitar”, they’re NOT a bluegrass group, just a jazz/whatever group that plays bluegrass.
I’m sure there are lots of similar examples of groups that would be called bluegrass if they did only acoustic sets but are ‘rock’ or maybe ‘alt-country’ or ‘Americana’ because they don’t have the traditional bluegrass instrumentation.
Mandolin is essential for bluegrass. Bill Monroe virtually invented the genre singlehandedly, and his mandolin and singing are two of its principal elements. No mandolin–no bluegrass. No high, lonesome harmonies–no bluegrass.
Occasionally, farm equipment.
Oh, you mean the banjo. 
Dobro is occasionally used in traditional bluegrass groups alongside the banjo/fiddle/guitar/mandolin/upright bass. The makeup of the standard bluegrass group was codified by Bill Monroe & The Bluegrass Boys. They invented the sound, and everybody else latched onto it. Then you get the occasional group with two fiddles, or a harmonica, or a simple drum kit (usually if you hear a drum, it’s a simple snare for a beat. Never a cymbal.) And sometimes you’ll get a banjo player who’ll swap out the banjo for something like a tenor banjo or something like that. If you go any “wilder” than that, traditional bluegrass fans consider it “progressive bluegrass”, “newgrass”, or I’ve also heard overly smooth and slickly produced bluegrass referred to as “NPR bluegrass”.
Jugs, washboard and cowbell are commomly used in bluegrass and old time string bands too.
I love that video so much. “Sweet Georgia Brown” arranged for guitars, bass and tractor, how can anyone not giggle at this? Hearing the delight of the video guy is almost as fun as the music itself. That’s a go-to video whenever I need a smile.
Sorry, there’s gotta be a whoosh here. How is a tractor like a banjo?
I think (s)he was lightly poking fun at the banjo for being obtrusively loud, clangy, and/or metallic-sounding.
And rural.
That. Was awesome.
FTR, of course, this is not bluegrass.
You’re right - one of them is loud and obnoxious, and the other is a tractor! 
Tell me about it! I just bought and modified an old golf trolley for use at festivals. Some time ago I gave up trying to keep the edges of my bass free of rash. Decided it just adds “character.” (Yeah, that, and I really WAS trying to get through that dorway without clipping it…)