Simply go to Settings > Invite Someone > and share the invite code. They’ll use the invite code, and join the house. You or they can then edit costs to involve them in the recurring bills. To pay their share, they simply need to add a card by going to Settings > acasaPay > Add a new card.
You can only remove someone from the house when they are; no longer involved in upcoming costs, not owed or owing money. You can remove them from the upcoming costs by going to Spending > cost > edit > and unticking them from the cost. You can use acasaPay or register an external payment in order to bring them to being all squared.
To be more exact about the amount owed you can customise the cost splits, using the percentage splitting capability. You can also send us a meter reading for your energy (and water if you have it), and you can use this to work out everything to the penny.