Hey @ariche 
I’ve moved your post here - where Joint Accounts have been discussed at length. There are no plans to charge for accounts currently.
This is half right - Monzo do indeed plan to offer overdrafts and will earn interest on those. However, plans for ATM fees abroad are yet to be announced. Should the 3% fee be introduced that only kicks in after £200 has been withdrawn over the period of a month. The 3% would then offset those free withdrawal costs. However, this is very off topic and I would suggest any conversation about ATM fees should be moved here: Monzo - ATM fee chat
Monzo are also transparent about how they plan to make money - the Marketplace being one of them.