What if one roommate pays and another does not?
The paid share is recorded as paid for that roommate. The unpaid share goes late on its own, that roommate is charged the proportional late fee, and the rest of the household is told. Because every tenant is jointly liable, the landlord may still collect the unpaid amount from any of them.
From the guide Splitting Rent with Roommates.
Everyone is still responsible for the full rent
Splitting the rent is a payment arrangement, not a change to who is liable. Under the lease and the addendum, every tenant is jointly and severally liable for the full rent: if a roommate does not pay, the landlord may collect the unpaid share from any of the others. The household view on the tenant dashboard shows where each share stands so nobody is surprised.
Read the whole guide: Splitting Rent with Roommates