Well what happens at newcastle is they hold trials in the first two weeks, which will probably happen at Northumbria also. There will be trials for the actual University team and then proabably trials for teams that are intermural teams, which contain university students but arnt actually the university team and these play against each other in various 7 and 11-a-side leagues. Also like you said there will be 5-a-side teams. It should all be well documented during freshers week, with teams advertising for trials trying to get players in. It all depends on how seriously you want to take it as to what you go for. For example if you want to take it pretty seriously and train a couple of times a week with competitive games then you should try out for the uni, If not then maybe another team like ive mentioned. I can't say that all this is correct because i don't go to norhtumbria but that is how it works at newcastle.
To be honest, i wouldn't worry about it as during freshers week there is loads of information about stuff like this and its easy to get involved. Hope this helps.