Well...

we know that Anna Lucia's group were attacked and Infiltrated by goodwin.
He and ethan were to return in 3 days to henry Gale. that mean acting as if they were there from the moment of impact and blend in in the confusion, get info ( the names) might take a while as people are hysteric. and then come back 3 days later. It wasnt going to take them a day and a half to hike there and then get a quick list of names for 5 mins and then leave right away for another day and a half hike back. Nah.... and they have boats...!

Much more likely that They are one big group and are keeping our hostages seprated from the teachers and kids area. (thanks lizard) henry Gale and the others have boats and guns and technology. I hardly beleve they would be allowing YET another rogue group live on the island and carry out such similar behaviours... economically speaking. thats not condusive to getting the job done. They obviously have some job or goal they are trying to complete.