A few things to check:
Ask them to check their spam folder Invite emails occasionally get filtered. Have them search for an email from subvert.fm.
Resend the invite You can resend a pending invite from your Collaborators dashboard. Find the pending invite and click Resend.
Check that the email address is correct If the invite was sent to the wrong address, cancel it and send a new one with the correct email.
The invite link may have expired If the link in the invite email has expired, resending will generate a fresh one.
If none of the above helps, contact us at info@subvert.fm and we'll look into it.