I had an idea for a “game” that a bunch of musicians could play, which I call Ouija. The jam group is weak rhythmically, so I came up with this in order to improve our rhythm. A game should probably take 20-30 minutes. Here’s the idea: one musician starts by playing a repetitive pattern or motif. Other musicians gradually come in with their own complementary repetitive motives. Musicians can drop out, abruptly change their motif, change instruments, or whatever at any time. The only requirement is that each motif a musician plays must be played for a “long time” in some sense of the word, except for occasional short fill-ins. That is, if you asked somebody to describe the line of music that was being played, they would probably say “repetitive”. Oh, and the motives have to be “in time”.
My hope is that this will give us practice playing in time with each other, while making a musically interesting song. I’m guessing that the music will gradually change between moods.

When i faced that problem with friends that was usually best resolved by simplifying:
make everyone shut up more
make everyone who is not taking turns being lead do something boring that requires no thought, so that they can fully listen to the lead. eventually move towards responding to the lead