As a Player, there are three ways to claim your character, all 3 of which require that your GM first registers with Scabard. If your friend is not on Scabard yet, you can invite them from here.
Once they are registered, the 3 ways for you to join their campaign are:
Player initiated: Find your Character in your friend's campaign (while logged in) and click the "Claim" button. This sends your GM an email. He or she will then click the "confirm" link in the email. You're now the player of that character!
GM initated: Let your GM know your username and they can use the connections editor and add your username (not your real name!) to the "player" field. This field is case-sensitive.
Power Player (If you GM is a Hero subscriber and has assigned you as a Power Player): Go to your character's page and assign yourself as the player. If your character doesn't exist yet, create it first.