They may be affected by one of the following reasons:
1) Their device could be using an old OS version. The app will only work on iPhones running iOS 15.0 and higher, and Android phones running version 8.0 and higher. Also, some smartphones may not be compatible. Please check the version of the device's OS. Alternatively, they can use the browser version. That should solve most problems.
2) A maximum of 100 people (including you) can join one album. You won't be able to send invitations after reaching the limit. To make room, you can delete some accounts by going to "Family Members" in the Settings tab.