Marina,
By the way, I've found out that you cannot invite people to a
subgroups.
True, because they must first be members of the primary group. You invite them there.
You can either add them (which is a Premium Group feature, if I am not
mistaken) ...
Not for subgroups. Even in Basic groups subgroups have a Direct Add page. There you see a list of the primary group members with a checkbox to add them to the subgroup. What you can't do (not even in a Premium group) is Direct Add someone into a subgroup who isn't already in the primary group.
or they must joing the group on their own (through the subscribe
address, I presume).
Or, in the Admin section of the primary group there is a Subgroups page which also shows a list of the primary group members, with checkboxes for each subgroup. There you can conveniently see which subgroups any given person is in, and change them at will.
Shal
Shal Farley