Why do links in system PCP welcome emails fail?
I think you may have two different issues here.  I left a comment for one.

As for the lack of account signup - go to www.yoursite.org/admin/config/people/accounts (or, if you have standard Drupal menus, Configuration menu > People > Account Settings.  Under Registration and cancellation, Who can register accounts - ensure that the value is set to "Visitors" or "Visitors, but administrator approval is required".  If it's set to "Administrators Only", the account signup won't appear on the profile.