Set the flat up once
Create an account for the flat, pick its currency and type your flatmates' names. They don't have to sign up on day one: they're added as guests, and you can send everyone a read-only link to the balances. When someone does sign up with the email you invited, they take over their place and everything recorded for them.
Rent and bills that post themselves
Add rent, internet or a streaming subscription once as a recurring expense (weekly, monthly or yearly) and Hisaab Kitaab records it on the right day. If the rooms aren't equal, split by shares (the big room counts as two) or by exact amounts, and it stays that way every month.
Groceries and everything in between
Anyone can log what they paid in a few seconds. Split equally between everyone, or only between the people it was for. Attach the receipt photo if someone asks later, and discuss an expense in its comments instead of the group chat.
Settling up
Hisaab Kitaab nets everything out and suggests the fewest payments that leave everyone even. The person who owes sees your payment details (bank, PayPal, Venmo, Revolut, UPI and more) with a button that opens the app. When they record the payment, you confirm it. A friendly reminder is one tap away, and a weekly email digest is optional.
When someone moves out
A flatmate with a zero balance can leave the account; the history stays. When the tenancy ends, close the account: everyone gets a statement by email with CSV files, and nothing can be changed by accident.