I’m on 3.3.0.beta2-dev c77d109362
I am an admin, but I have added myself to the groups. It’s both the sidebar and the mailbox that the group mailboxes don’t appear.
The + that you pointed out at the bottom of the sidebar looks like it’s for adding custom sections, not for making mailboxes appear. I also checked the + next to the “Messages” heading, but that’s for composing a new message.
I also asked a non-admin user who is in one of the groups that I sent a test message to, and they get the same behavior. They can get to the inbox using a direct link, and they get a notification for receiving the message, but they don’t see links to the group mailbox anywhere in the ui