Send a katch as an invite
You can quickly add a contact by sending them a katch. Tap the "Send" icon, tap on "Invite someone new" and you can select a contact from your address book to send a katch that will invite them to use katch.
Send an Invite
You can send an invite without attaching a katch. Tap on the profile menu, select "Invites" and then invite a user in your address book.