If you received an invitation but the group album does not show up in your list where it says "My Albums", try the following steps:
First, go back to the invite e-mail or SMS and click on the "View Photos" link directly. Then, on the web page that loads, sign in to associate the group album with your account.
When you go back to the albums list on your phone, this new album should appear in the list. (Leave the app and come back to refresh the list.)