When someone else on JamKazam invites you to join either a public or private session, if you have the JamKazam app open, you will see a notification message pop up from the bottom center of the app screen (pictured below with arrow). You can click the orange Join Session button in the notification message to join the session.
If you see the notification but aren't quick enough to click the button before the notification message disappears, you can still click the Notifications bar in the blue column on the right side of the page to see all your recent notifications. Any session invitations for sessions that are still active will be displayed here, and you can click the Join button to join the session from here (pictured below with arrow).
One other handy feature you can use to see and join sessions to which you've been invited is the Find Session feature. To use this, click the orange Find Session tile on the home page of the app (pictured below with arrow).
The Find Session page is displayed, with the "For Me" option selected/active at the top of the page. This shows you sessions to which you have been invited, as well as any sessions started by your JamKazam friend connections. If you see a session to which you've been invited on this page, you can click the Join icon (pictured below with arrow) to join the session.
When you click a join button or icon to join a session, you'll see a Terms and Conditions dialog box. Click the orange Accept button (pictured below with arrow).
You are dropped into the session (pictured below).