Thanks for the suggestion! I’ll pass it on at our next design meeting.
For now, if you’d like to quickly view only the people you have balances with, just tap the “you owe” or “you get back” cell above the list of people (where it lists your total balance), and the screen will update to only show the relevant people. Hope that helps!
We’ve been considering allowing friends without email addresses, though we haven’t yet come to a decision. For now, you could try creating a group with this person in it (groups allow names without email addresses), or adding the person with a variation on your own Gmail address (e.g. firstname.lastname@example.org). Gmail ignores everything after the “+”, and those messages will end up forwarded to your own Gmail account.