1) Can the address? [email protected]? also be used for the main group?
?Yes.?
2) Can the address? [email protected]? ?also be used for the sub group?
?No.?
Subgroup names need only be unique within the group, so they can't be used in a format without the collective group name (XXX).
Note too that "main" is just a default name for the primary group. You can change it if there's something that would make more sense in the context of your group. See also this description: