The Group command creates or updates a "group" object. The grouped objects are collected underneath a + sign in the object tree (see images).
Group is tremendously helpful with multi-state or multi-structure sessions. Wildcards work great, for example:
# put all of objState into the group "ensemble". group ensemble, objState*
group name, members, action
group efHand, 1cll 1ggz 1sra # allow addition and removal from the group group efHand, open # disallow addition/removal from the group group efHand, close
Group objects can usually be used as arguments to commands. It can be processed as a group or as a selection, in which case all the atoms from all objects in the group will be used.