fix(Core/Groups): Create the group at first invite (#17869)

* cherry-pick commit (https://github.com/TrinityCore/TrinityCore/commit/132538db1dfb3c495294eeb5acfead59dea02b5c)
* cherry-pick commit (https://github.com/TrinityCore/TrinityCore/commit/250e8d998bca7f239e2393b6d002441f70d4daf0)
* part https://github.com/azerothcore/azerothcore-wotlk/issues/11070

Co-authored-by: Treeston <14020072+Treeston@users.noreply.github.com>

src/server/game/Groups/Group.h

@@ -243,6 +243,7 @@ public:
     GroupReference* GetFirstMember() { return m_memberMgr.getFirst(); }
     GroupReference const* GetFirstMember() const { return m_memberMgr.getFirst(); }
     uint32 GetMembersCount() const { return m_memberSlots.size(); }
+    uint32 GetInviteeCount() const { return m_invitees.size(); }
 
     uint8 GetMemberGroup(ObjectGuid guid) const;