I just checked your account and your traffic limit looks fine. You've only used 4.3 GB out of 22.4 GB and you're doing pretty well on storage as well (1.4 GB of 2.2 GB used). Assuming that all of you are using clients with Free licenses you have a 2 GB client-side sync limit.
The client-side sync limit means that the client itself cannot sync more than 2 GB, no matter how much storage you have available or where it is. That means if you create a Space and store 1.4 GB in it and you classmate creates a Space and stores 1 GB in it, if you join that Space the total client-side sync is 2.4 GB which is above your client side limit.
If you buy a license the client-side sync limit is lifted for that user. Licenses are user specific so one user can have multiple installations. If you're in a group of 8 people and each person has created a Space that is 1 GB, everyone will not be able to join the Space because they will reach the client-side sync limit when joining.
How much data, total, are you all trying to sync?