Yeah, some structure is required. Experienced improvisers can sometimes wordlessly communicate a structure. But even experienced people allow themselves to have a chat before starting to play.
Try playing songs! OK, strictly that might not be "jamming", but it's satisfying and it can put the skills in place for jamming. Print some chord sheets with a nice clear song structure, and play through it together from beginning to end. If you don't play songs, it's all too easy to just play the same riff for half an hour, or the same 12 bar chord progression.
By "song" I don't mean someone has to sing (although it's good if they do). I just mean something with a beginning, middle and end; a sequence of different parts. It's satisfying to perform something finite. Then you can talk about how it could be improved, and try it again.
Within that structure, there's room for improvising. "OK guys, how about we do the first two verses, then the chorus, then the third verse, then we'll do the chords from the verse, and go round the room getting a solo from everyone who wants one, then a final chorus, then the end".
Other structures you can impose are things like "trading fours" -- some number of musicians provide a backing, while two or more people take it in turns to play four-bar licks, ideally taking cues from each other. At first you'll need quite quite a strict rota ("You two trade fours for 16 bars, then go back to being rhythm players and the next pair can trade fours"). Encourage everyone to use eye contact to help each other keep to the structure.
As everyone gets more experience, you can be more loose ("You two trade licks until you're ready, then you two take over") -- it doesn't need to be a strict 16 bars any more, because they know how to communicate the handover.