If you're new to jammr, try joining a public jam session first so that you can
familiarize yourself with jammr and chat with experienced users if you
have any questions.
After downloading and installing jammr
(https://jammr.net/download.html), launch it and log in with your
username and password.
The “Connect to server…” dialog will open and allow you to join an
existing jam session. You can disconnect from a jam and connect to a
new one by going to File | Disconnect and File | Connect.
Premium accounts can join and create private jam sessions so you
can play together with just your friends. Free accounts can only join
public jam sessions.
If you are a Premium user you can create a private jam session by
clicking the “New jam” button in the “Connect to server…” dialog. You
will be in an empty jam session and can control who is allowed to join
by going to Admin | Access Control. Once you've entered your
bandmates' usernames they will see the jam session and be able to join
when they open the “Connect to server…” dialog.
You can also set the jam session's title, change the tempo (BPM), and
set the chord progression length (BPI) from the Admin menu.