If someone has invited you to join the group using the wrong email address -- for instance, your work email instead of your personal email, it's easy to change the email address on your account. Here's how:
1) Log in to your Pay By Group account
2) Go to the "My Account" page from the drop down menu in the upper right
3) Type in a different email address
4) Click "Update my account"
Make sure you double check that the email you're entering is accurate. If there's a typo in it, you might not receive your purchase receipt or other important information from Pay By Group!