I moved back to FL and found myself in an area that was quite far from any jams. In SLO, I had several jams available to me weekly and I sorely missed them. I am not a leader or organizer type, nor am I a particularly accomplished player and certainly no more than a mediocre singer, but I quickly realized that if I wanted to play with others I needed to start a group.
As for getting members, I found meetup to be the easiest most effective method. Lots of people already use this website and it is also an easy way to communicate with members. It does cost a bit but you can set it up to take small donations to keep it going. Others use “Next door____”, which is free but much less well known. You can also use the social part of craigslist and its free but far less folks find it. Signs up in music store or libraries also work but for me “meetup” has just been super easy. Now that we have lots of members, we don’t really need it and could move to something like a facebook page, just to keep in contact with members, but I don’t mind the expense of meetup for the convenience I get.
As to venues, we play at a large gazebo in an underused local park at no cost to us. We are lucky to have great weather and folks enjoy playing outside. I think the bit of hambone in all of us likes to be out in public (another group about 1hr away plays at a beach pavilion and is also super popular, probably partly for the same reason). Alternatively, here in Florida, we are rife with old folks (like me), and lots of folks have clubhouses in their community or in some parks (for a fee) where you can play in a more private, climate controlled situation. I have always liked the use of a bar or casual restaurant as a convivial setting to make music, but this will take a little more effort to work out. There is usually a place that has some slow times and would be open to having someone. The one hassle that kept me from doing this is the potential for problems with playing non-original material. Being in a tourist area, the likelihood of enforcement might be greater than one would expect. People have mentioned libraries but I am sure we are too raucous for that venue.
As for music, I select it (with group input) and put it in “dropbox” for folks to access. We go around the circle in order and select a song to play. My group is an open acoustic jam for all instruments but with a solid core of ukers. I like the mix. Its more interesting. Other local groups use “The Daily Ukulele” which is certainly an easy way to go, but I find it more interesting to have a different mix of music. There are lots of pre-made songbooks from existing groups, “Ukulele Wednesdays” (London based in pub, free download) is an excellent example.
Good luck with your group. If I ccould do it, so can you. Be sure to make it fun for yourself, otherwise what’s the point.