The primary account on Sunbound membership is ultimately responsible for the full amount due. Any missed payments from other individuals you are splitting the bill with be charged to the primary account holder.
For example, if your bill is $1,000 and you split the bill equally with one other individual, so you each owe $500 ($500 times 2), if the other individual does not pay by the 25th in the month that amount will be charged to your account. Therefore, you would owe the full $1,000 balance.
Reminder - you must pay your total due in full each month or you will be unable to access Sunbound's Pay Over Time to pay your rent in the following month.