Are you allowed to bring outsiders to freshers?

When people say they are going to freshers, they often mean going to the club nights put on by clubs in town that aren't the SU. It's pretty rare for these clubs to even be officially "Students only" on these nights, although some do propper student nights and will ask for student ID (but most would accept an NUS card anyway).

Usually, your SU will let you sign in guests, but numbers may be limited at fresher's events.

Most people I know go out during freshers week in the city they live in, even though none of them are freshers and a lot of them aren't even students. It's just a fun atmosphere, and there are usually drink deals on.