@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