When you send a user invitation, it is automatically generated by CrowdWorks. This means that the sender of that email is hello @ crowdworks.it + your name.
For example:
"Amanda Johnson (via CrowdWorks) <[email protected]>"
Check you have sent the invitation to the correct email address
The invitations subject line is "Amanda Johnson has invited you to your company name". You should suggest that your user searches with that term in their inbox
Guide your users to check their spam folder (it can also end up in promotions, socials, or forums in Gmail)
Make sure the user has not blocked "hello @ crodworks.it" by accident
Withdraw the old invitation and send a new one