Ask people in person, around your neighborhood, at church, at work, etc., if they play or know someone who does. When you run across someone who plays (or sings, if that fits your profile), chat briefly about what kinds of music they like to do, and try to get a feel for their attitude about it. For example, a bluegrass purist may not care for sessions that include rock (and vice versa), but some bluegrassers would be fine with, say, folk or 50’s rock. Obviously you want some compatibility with what you like to play. I gather from your post that you also would do best with mellow frames of mind that are accepting of different genres, different skill levels, etc.
For the written notice on bulletin boards, try to give folks an idea of what kinds of music, skill levels, which instruments, etc. you’d like to include. Have them call you to review it in converstation.
If you can get one or two or three interested for starters, invite them over for pizza and beer and see how it goes. If there’s compatibility and everyone enjoys it, set a date for the next session and have them all spread the word. These things can grow pretty readily once they get off the ground. Someone may have several friends who play that can be invited, or may invite you to a session where you can meet others who might be interested.
Some sessions make a point of passing the turn to pick a song around the circle, to make sure everyone gets a chance to do so. Other sessions seem to be rooted in a common, often narrow, repertoire where someone says “let’s play this” and it’s expected that most of the others know it to some degree, with the idea of enforced turns not really part of the deal. Think about which way is better for what you want to do, and guide it in that direction.
If you get a group going, regular monthly sessions usually work pretty well. It can have one host (presumably you) all the time, or the group can take turns with a rotating hostship. Once you get beyond a handful of people, BYOB (plus bring something for the snack table occasionally) may be more feasible than the host providing everything.
The main thing to get started is to find people to ask about joining you. Besides the other suggestions in this thread, you might look for folks who play at local open mikes, or local music events, make presentations in schools, etc. Good luck!