Translation:The girls return to the restaurant and sit back at the table.
"sit back to the table" would not be acceptable in the English I learned, and "sit back at the table" sounds wrong to me as well - "sit back down at the table" is what I would say or, if you are not talking about the movement towards the sitting position but about the resulting sitting position, "sit at the table again".
Not a lot of difference. Visszatér is a more general "to return", and visszajön is "to come back", i.e. spoken from the viewpoint of the place they are returning to. If you're outside of the restaurant and watch the scene, you cannot say visszajön, but can still say visszatér. Or visszamegy in this case.