Core/Groups: Prevent group id pool corruption (BG groups take their Ids out of the same pool thus the need to register properly)

src/server/game/Battlegrounds/Battleground.cpp

@@ -1203,6 +1203,7 @@ void Battleground::AddOrSetPlayerToCorrectBgGroup(Player *player, uint32 team)
         group = new Group;
         SetBgRaid(team, group);
         group->Create(player);
+        sObjectMgr->AddGroup(group);
     }
     else                                            // raid already exist
     {