Joining a group is simple! If there is an invite on your Groups screen, tap on the Join button next to the group name. If not, press the large Join Group button in the top right corner and enter the 4 digit code for the group you want to join, then tap the large Join button. The code is found on the top right corner of the Current Group screen of any member. You'll see a Group Joined screen. If joining a group over Bivy Stick, this may take several minutes. You'll then see your current group when you go to the Groups tab.
When someone joins a group, the first time they open the app with service all previously sent group messages will download to their phone.