Add sender's address to warning on JoinRequest to be able to identify it.
Introduced the MemberGroupType.PER_MEMBER instead of relying on an obscure null value
Updated the documentation to include MemberGroupType.PER_MEMBER
Merge pull request #649 from pveentjer/improvement/3.0/member-group-type-per-member

Improvement/3.0/member group type per member
Merge pull request #646 from MrEasy/patch-1

Add sender's address to warning on JoinRequest to be able to identify it...