Indeed, what @nthn says; either someone adds you to an existing group, or they have invited you. In the second case, you can accept the invitation from Signal Desktop. If you can’t do that for some reason, you can ask them to add you again (but that hasn’t been tested by anyone afaik).
If everything moves smoothly with the people that are currently testing, and when I’ve fixed the above four bugs, I’ll pull out beta.1 by the end of the week.