In BeeBEEP 5 an administrator adds you to a group and you are in. In the new BeeBEEP you are invited, and joining is your choice.
The invitation is a message
An invitation arrives in your private chat with whoever invites you, like any other message: the name and color of the group, its picture, how many people are in it, and a few words from the inviter. It never carries the list of members: until you accept, you learn nobody's identity and nobody learns yours. If you are offline, it waits and reaches you when you come back, days later if need be. An invitation can be withdrawn, and it expires after a month.
Accepting means introducing yourself
Joining a group means accepting that its members learn who you are. So when you accept, your answer is signed with your key and carries your identity card. The administrator checks the signature and adds you, then sends every member your card, introduced and signed by the administrator. From that moment the others can write to you with end-to-end encryption, even those you have never met on the network.
A clean start
When you join, the conversation shows "You joined. Earlier messages are not available.": what was said before you arrived stays with those who were there. Declining is simply an answer; ignoring an invitation is fine too.
Colleagues still on BeeBEEP 5 cannot answer an invitation, so they are added to groups directly, as they always have been.