It is possible that their confirmation/welcome email could get spammed. It comes from firstname.lastname@example.org so please ask them to double check for messages from that address, and to also check their spam folder.
If no luck, it is still possible for them to set-up their account without the missing welcome email:
1) Have them go to MissionMinder.com and click "Member Login" top right
2) Click the "Forgot/Reset Password" link
3) Enter their email address and click the "Send Password Reset Email" button
This will then automatically walk them through the steps to get their password and account access.