use the given user's login group.
If SPEC_ARG contains a `:', then use that as the separator, ignoring
any `.'s. If there is no `:', but there is a `.', then first look
use the given user's login group.
If SPEC_ARG contains a `:', then use that as the separator, ignoring
any `.'s. If there is no `:', but there is a `.', then first look