If the user you're trying to invite has not received their activation email, first ask them to check their spam folder.
To resend an invite link:
Click on Configure
Select the Users tab
Search for the user you'd like to invite again
Click on the 3-dot menu on the right
Select Send invitation
Alternatively, you can also click Copy invite link from there and send it to them directly:
Need a hand? Contact our friendly Support team via the in-app chat at the bottom-right of your screen, or email firstname.lastname@example.org.