@ADD <Object1> = <Object2>

Creates a membership association between <Object1> and <Object2>.
Order is unimportant.  One object must be a:
  Player, Room, Exit, or Item which you own or control.
The other must be a:
  Group, Sticky-Exit, Copy-Object or Owner-Group
  which you must own, control or have shared access.

Being a member of a group or carrying an object that is a member of a
group will permit you to successfully use objects/exits that are locked
to that group, or fail to use them if the object/exit is locked against
that group.  Group memberships can be checked by the containment operator
in locks.

See also: @remove, control, copy, groups, locks, membership, objects,
    operators, owner-groups