I have setup LDAP for authentication in the past, although I have not done so in about 18 months. I know that the solution consulting organization has step-by-step guides for how to do it. You can ask your account executive to get the latest copy.
The closest I can find on the PDN to get you going right now is the "Authentication Using LDAP" lesson from "Module 9: Security" of the Senior System Architect course from Pega Academy. I believe that all Pega Academy courses are free so you should be able to access the content. The direct link to the content for me was https://pdn.pega.com/node/1011711.
You will need bind user and password to connect to LDAP. You can go through the OOTB activity AuthenticationLDAPVerifyCredentials for reference, which uses search filter to search for the given user. The search result will give the user attributes as well. Post that, it does the password verification of the given user. I think you can use the search part of it for your requirement.