You need to decide if your jam session is an open jam or an invitation-only gathering. A jam session is different from an open mic night because all the musicians are expected to play a common song together. Usually when given the choice, musicians prefer to play with people who are at the same level or better than they are. Jam sessions that are invitation-only are usually of a higher quality than open jam sessions.
Jam sessions usually have a theme, such as blues, jazz, rock or Latin. The theme helps draw out musicians who have that particular style. Generally, people who play the same type of music have overlapping repertoires. This makes it easy to choose songs and gives the impromptu band a rehearsed sound. Choose a list of the most popular songs or standards of that particular genre so you can refer to it later and make suggestions.
The amount of people who show up to the jam session will determine how long each person gets to play. Usually, the supply of musicians will ebb and flow during the jam session, and you will have to adjust playing times accordingly. The rule of the jam session is to let others play too, so respect the rotation and do not hog the stage. If you have a unique instrument, like the mandolin, you will probably get more playing time. If you play a popular instrument, like the guitar, there will probably be more people with whom you will have to share stage time.
A good rule to follow for jam sessions is to respect the level of other players. If you are the best musician in the room, it does not benefit you, except during solos, to play at a level unattainable to the other musicians. Equally, if you are the worst musician in the room, a good rule is to keep your part simple and make eye contact with the other musicians so they can help you.
If your relationship with your neighbors is so bad that you do not want to talk to them about your jam session, you probably should hold it somewhere else. Jam sessions are, as a rule, noisy. Show respect and consult with your neighbors. Let them know you will be holding a jam session and tell them when it is and how long it will last. If they have a conflict, it is in your best interest to try and find another time that works for everyone.