I went to Music Core before for SNSD when they had their debut performance for RDR. If you want to be in the general audience, then it is on a first come first serve basis, and they usually start lining up around 12pm. If you are going for a specific artist, like I did with SNSD, you get to attend both their rehearsal/pre-recording, as well as the live show. However, they're recorded at different times, and for the more popular artists (SNSD, Big Bang, etc) they usually start lining up around 7. Look for the person in charge of the group and ask if there is a list so that they can add your name for the artist. Usually the group lines are handled by the fan clubs and they're super organized. In order to attend a taping for the group you usually -at least- have to have their current album or single they're promoting. For SNSD we had to have that as well as their song on our phone and their image as our phone's background. I think that was because it was their first performance of the song though.
As for the shows, Music Core does not allow you to record the shows. They will actually take you camera from you and delete any and all footage. I remember Inkigayo and Music Bank allowed you to before, I don't know if it's the same now though. You can always check though.
If you have any other questions, feel free to ask~