When an invited user tries to register but gets a message saying their invitation can't be found, it usually means the system can't match the email they're signing up with to an active invitation. On your League Page, check the Pending Accounts section to make sure their invitation is listed and that the email address is an exact match.
It's also worth checking the list of existing users—sometimes the person has already registered in the past. And if you sent multiple invitations, remind them to use the link in the most recent email, since each one includes a unique secure code that must match the system during signup.