Users with access to a place via group cannot access the place while offline, but can still access place via server.
Resolving the problem
To use a place offline, users must be individual members (directory or local) of the place and any rooms. A user who has access to a place or rooms through a group membership (directory or local), cannot use the place or rooms offline.
Enhancement Request 3507 has been made asking that offline functionality be available to group members of places and not just individuals members of places.