Link contacts by Phone Number
I'm sure you have usage statistics on this, but I'm guessing the primary users of your app are phone-based, and not PC-based (except for features currently not possible in the apps).
Due to this, contacts should be linked by phone numbers, and debts can be sent by push notifications (another idea I've posted). That'll allow easier addition and maintenance of friends.
You can now invite friends by phone numbers and receive push notifications. There are still some issues with matching invites to users that we are working on.
Namith Krishnan commented
Instead of using email for signup please use mobile number. We don't know each and every friends email id
Katie Hart commented
Is there a way of finding friends that already have the app? I have friends that have said they have downloaded it but I can't find anywhere that prompts me to add contacts, only to invite them to join - which they've already done!