DON’T have a joint account with housemates. As mentioned above, you’re financially linked at the Credit Reference Agencies not only for the time you hold the account, but for six years afterwards too, so if your friend decided they want to wreck their credit file, they’ll wreck yours too.
If Monzo create shared pots, somehow, which aren’t legally linked, then great, but the prototypes/concepts we’ve seen before have flaws in that only one person really controls the money, despite anyone being able to contribute, meaning unscrupulous people could just take the funds. Even if withdrawal rights are delegated, there may be probate implications if the controller of the pot dies suddenly.
It will be very interesting to see what Monzo invent. Personally, I’m sceptical about the legal ramifications of shared pots, and would probably consider a powerful bill splitting app.