Two things you can do:
1. Resend the invitation: Go to Accounts > Members > Project. You can resend by clicking Action > Resend.
2. Ask the recipient to check their Junk/Spam folder. If so, have them transfer the email to their Inbox. If a comment appears asking to report the email as Not Junk/Not Spam, click "Yes". This will improve their ability to receive your TruCentive deliveries.