Re: Can't join Sims 4 university organisations

@Laurathehefty 
Sounds like you have a good start by going to the gatherings on campus. 
Are you on PC/Mac or Console? It should act the same but troubleshooting may be different.
I must ask if your Sim is actively enrolled in University and attending the gatherings for that campus.
As long as it is an active meeting / gathering / activity there should be a "title" saying the Sim is a "Member of ..." what ever organization is gathering.  I have found it a bit tricky at times and I would sometimes arrive at the event just as it ended.

Source:
Britechester & Foxbury Student Organizations
Participating in Campus Groups in The Sims 4 Discover University
https://www.carls-sims-4-guide.com/expansionpacks/discoveruniversity/student-organizations.php