You can do that by using the acceptance statuses of ConfTool Pro.
In this case, create a new acceptance status to discern between papers which are accepted and whose authors have already registered for participation and papers which are accepted and whose authors have not yet registered.
First, add a new acceptance status “Accepted and Registered” here:
Overview => Settings => Manage Acceptance Status
Mark the “Accepted” status and click on “Copy as new Acceptance Status” to copy the settings of this status. Rename the new status to e.g. “Accepted and Registered”
Then, assign the new status to the corresponding papers here:
Overview => Submissions & Reviews => Results of the Reviewing Procedure & Decision About Acceptance
Finally, use the bulk e-mail function to send the group with the relevant acceptance status a reminder:
Overview => Bulk E-mails => Send E-mails to Authors (and Co-authors)
Use the filter in the section “Acceptance Status” to discern between the relevant author groups.