To accept an invitation to an account you must first be invited. Once you're invited, you will receive an email with the invitation link. If you did not receive the email with the invite link, you may have not waited long enough (usually a couple minutes) or the email address could have been entered incorrectly.
To accept the invitation:
Open the invite email
Click the 'Accept Invitation' button
If you do not already have a FastBound login, you will need to choose a password
Confirm that you want to join the account by checking the box accepting the invitation
Click Accept Invite