Since you are required to create an account in order to join someone else's group album, you are automatically allowed to create a new and completely separate group album!
MOBILE: Just click on the back arrow in the upper left hand of the group album you were invited to. Once in your main screen, click "Create New" at the bottom and continue from there!
WEB: Click the "Other Options" icon (the three horizontal lines stacked vertically) in the upper left-hand corner of the screen. Once in your main screen, click "Create New" at the bottom and continue from there!