Groups and Topics can have their own Permissions. Students can join a discussion by opening a Topic directly, or by opening a Group and selecting from all the Topics within the Group.
Sharing a Topic Join Code will give Permission to that single Topic. Students must be given Permission to the Topic if they want to open it directly, without opening the Group first. Read more about creating a Topic and here. Topic Join Codes and Links are usually a random mixture of letters and numbers. Example: https://flipgrid.com/a591dc5d
Sharing a Group Join Code will give Permission to all Topics within that Group with a single Join Code. Students must be given Permission to the Group if they want to open it. Students who join the Group can then open any of the Topics within the Group, regardless of the Permissions specified on the Topic level. Group Join Codes and Links are commonly customized by the educator to make it easy for learners to join. Example: https://flipgrid.com/FGrid3567
Keep in mind: when sharing the link to a Public Group, all Topics within can be opened through that link - regardless of the individual Topic Permissions!