The simplest thing is to look for a JAM that is about to start and offer your services as a musician.
You have 2 options.
The help wanted subforum where you can offer or search for people who are requesting musicians. However, this is for general use, not related to any JAM.
The other option is that you review each JAM that interests you, when you enter the page of a JAM, you will see a tab that says "community", where you can write to offer your services as a musician. You must do this for each JAM in which you are interested in participating.
Many JAMs also offer a discord channel for people to communicate and there again, you can offer your services as a musician.