MOBILE: Go to the the group album you want to invite people to and tap on the "Settings" icon (this is a gear/cog on iPhone, and an icon with settings sliders on Android). Once in the Settings screen under "Members" it will tell you how many members are a part of that particular group album. If you tap "<#> Members" it will show you a separated list of "Joined Members" at the top (those who have accepted your invitation and are able to contribute to the group album as well) vs. "Invited Members" at the bottom (who have yet to accept your invitation).
WEB: At the top of the screen in the specific group album in question you can view the members of said group album by clicking on the "Members" button at the foot of your cover banner. The people listed under "Members" have accepted your invitation and are able to contribute to the group album as well. The people listed under "Invited" have yet to accept your invitation.