The “sixers” are people that came to Terra Nova on the sixth pilgrimage, some of whom splintered off and left the main compound to live on their own. We don’t know much more yet about why.
I think the latest group to show up, with the cop and the doctor family, is the tenth.
And to add to the ‘sixer’ intrigue, the leader of Terra Nova thinks that someone back in the dying timeline basically planted the ‘sixers’ to de-stabilize Terra Nova.