Technically it may or may not be against the universities rules. You can almost certainly look these up online. The keywords may be calendar of regulations, and governance.
The rules/policies of a university, like many large organizations, can be very fragmented. For example, when researching around this question, I found that there were no rules at my university preventing members of the general public from attending lectures, so long as there were enough seats. Where as there is a rule forbidding any exercise or sports being carried out in any location on campus not specifically assigned for such activities by the vice-chancellor (or his direct delegate). While the rules might not formally exist though, they may come under a internal "common law", where there is precedent for throwing randoms off campus etc.
With that said, on a practical basis there is likely little anyone can do about it, or even detect it, if you turned up in a common space (eg a collaborative study area in a library) and did what is practically collaborative study.
So long as you didn't disturb anyone, they almost certainly won't take note, or care.
If your group has any alumni, or current students or staff of the university or of a university this one has a relationship with, that would aslo decrease the chance of anyone having issue.
If you don't want access to any restricted resources (like private rooms or projectors), it is probably easiest to ask forgiveness, than permission.
If someone takes issue, they will most-likely just ask you to leave. Which of course you should do so quietly and politely.