If your member hasn't received their invitation to join the group, ask them to check their spam and promotions folders. It's also worth double checking you have the correct email address and/or phone number.
Occasionally we'll have bugs in our app which might impact your experience. Please let us know if this is happening, so that we can resolve them as quickly as possible at email@example.com.
These bugs can at times be related to old app versions. It's always worth checking that you are on the latest version of the app (Version 4.6.2 Build 202), which you can find by clicking in "My Account" > "Account Settings".