Figure out who's got the gear. From amps and instruments to cables and even small items like earplugs, someone has to bring all of your musical equipment to the same place. It's easier to create a jam with a pre-arranged studio space with an existing drum set so nobody has to wait for a drummer to set up tom and cymbal stands. Participants will still have to wait for others to power up their instruments.
Do sound checks to determine sound levels. See that musicians are comfortable with their sound levels to prevent "volume-ups" in mid-jam. Musicians who can't hear their instruments are known to crank up the volume gradually, and the result is a sometimes unwelcome escalation of sound. Make sure levels are set before you begin an open jam.
Start with general chord progressions. The host or jam leader can let everyone know what key to play in and whether there will be specific chord or key changes. Open progressions like a 1.4.5 let musicians work together to create unique sounds while following the same basic patterns.
Use easy, familiar cover songs for jams. Try songs you think everyone is familiar with as starting points for a musical collaboration. That way, you don't have to pick chord progressions; musicians who recognize the song will already know what they are. You can play the covers with or without vocals according to preference, set-up and whether you have a vocalist on hand.
Find ending points and breaks. Generally speaking, it's much easier to get a jam started than to end it. When musicians have found their groove with an open repeating progression, they tend to stick to it for a good long while. If your jam is based on a song, there's a definite end point, but if not, someone's going to have to create one. A host or jam leader can use body language to cue the ending riffs or "wind down" on their instrument to signal an end to a jam piece.