How to model a user/group relationship where users are assigned an unique number within the group?
After your comments, I think you need a simple many-to-many table with an additional UNIQUE constraint:

TABLE UserGroups
  GroupID 
  UserID
  UserNumber
  PRIMARY KEY (GroupID, UserID)
  UNIQUE (GroupID, UserNumber)
  FOREIGN KEY (GroupID)
    REFERENCES Groups (GroupID)
  FOREIGN KEY (UserID)
    REFERENCES Users (UserID)