I missed a lot of this series but see a lot of references to the 'insiders' and 'outsiders'. Could someone quickly explain this and who was in each group? Might start rewatching this series when the current one ends. Thanks in advance

It wasn't exactly rigid, but the general vibe was:

Insiders: Caroline, Conor, Aaron, Luke S, Ashleigh, Shivonne, Scott and Becky.
Outsiders: Adam, Luke A, Lauren, Deana

Sara pretty much got on with everyone. Scott and Becky were much closer aligned to the Insiders, but didn't really seem as much part of the group as the rest and both had numerous arguments with people in that group.

Amongst the outsiders, I always thought that Adam, Luke and Lauren seemed to have a more genuine friendship with each other, while Deana wasn't quite on the same level. (Though they were always nice to each other)

I would consider Scott more of a tweener between the two groups, Becky was full on a member of the insiders by the end of the show, especially after Conor stayed over Shievonne.

I would also consider Lydia one of the outsiders as well.