Implement UX for creating a private group

Depends on #648 (closed). Subtask of #127 (closed).

Added ~14 and removed ~15 labels

Reassigned to @grote

Started branch 661-implement-ux-for-creating-a-private-group

Added ~8 label

mentioned in merge request !353 (merged)

Maybe we should split up this ticket into the creator's side and the invitee's side.

Good idea!

Added ~11 and removed ~8 labels

Status changed to closed by merge request !353 (merged)

Status changed to closed by commit e8c48ccf

Added ~10 and removed ~14 ~11 labels

mentioned in commit e8c48ccf