Save together, and see it.
Ajo, Adashe, Esusu - the way your group already saves. Mutara keeps the record, so nobody has to remember who paid and who did not.
The ledger is the product.
Every payment is recorded once and can never be quietly edited or deleted. A correction is a new entry with a reason, and everyone in the group can read it.
Reminders nobody has to send.
Mutara tells each person when their turn is due, who has paid, and whose turn it is to collect. You stop being the one who chases.