We've got Moodle 2.2
In the past, we've had WebCT/Blackboard & it was very easy, in the group creation tool to find all students not in Group 1/2/3 etc - so, either those not in any group, or those not in a particular group.
From what I've seen at the moment, it's not so easy in Moodle. I can see who's in a group, but not who's not.
Searching this forum, I saw a post from about 2007, asking why that feature had been removed from 1.7 ... but guess I can't be the only person to have wondered it since! (I'm speaking as someone who uses it at teacher, rather than administrative level)
Wanting it for a couple of reasons - 1 to create department level courses - then group students within them on a year basis (from looking at http://docs.moodle.org/22/en/Groups_FAQ#Can_I_automatically_add_users_to_a_group_at_the_same_time_that_I_enroll_them_via_a_csv_file.3Fit seems that it might be possible to ask for that when they're enrolled) [Just been told that's not possible, due to the fact they're using LDAP sync]
The other reason is that students work in groups & I use the group function to record who's in a group etc. I need to know who to chase when they've not told me which group they are in!