If a family member is using the browser version of FamilyAlbum but wants to try the app version, you'll need to send them a new invitation. To do so, go to the Settings tab, tap "Invite Family" under the family members list, select their role, then tap "Invite to App Version".
This will create a separate account for them to use the app version of FamilyAlbum. Unless you delete their browser-version account, they'll still be able to access the browser version of your album.
If you'd like to delete their browser-version account, go to the Settings tab, tap their nickname under Family Members, then tap "Delete Account". This will disable the URL they used to access your album via browser and any email notifications for new uploads/comments. Even if their browser-version account is deleted, any comments they made using that account will remain visible. Only Album Admins (parent members) of an album can delete accounts.