Any entity may be used as either a group, or as group content. To enable a particular bundle of an entity type to function this way, go to admin/config/group/fields.
- Select the entity and bundle
- Select the field to add:
- Select 'Group' to make entities of this bundle able to function as groups
- Select 'Group audience' to make entities of this bundle able to function as group content
Note that a limitation of the current UI does not allow it to make more than one entity type into a group. However, the relevant fields can be added manually in the entity bundle field admin.