The PTA Member Directory plugin and the PTA Volunteer Sign Up Sheets plugin are two separate plugins. If you want to have volunteer sign ups, you need the PTA Volunteer Sign Up Sheets plugin:
https://wordpress.org/plugins/pta-volunteer-sign-up-sheets/
Documentation for sign up sheets plugin is here:
https://stephensherrardplugins.com/docs/pta-volunteer-sign-up-sheets-documentation/
The member directory is a simple directory & contact form listing that I created for my son’s school PTA site several years ago. Yes, an admin needs to enter them manually. They are not associated with user accounts. Just a simple way to organize and list the members of your organization and provide a contact form that can be used to contact either individual members or to send an email to all members of a “position”. Documentation is also online here:
https://stephensherrardplugins.com/docs/pta-member-directory-contact-form-documentation/