Where does WebSphere Portal 8.5 store user registration data that is entered through sign up form? I.e. user id, password, first, last name, email etc.
WebSphere Portal doesn't store any user info on its own.
It uses PUMA and VMM to talk to users store registry. See more here http://www-01.ibm.com/support/knowledgecenter/SSHRKX_8.0.0/plan/plan_vmm_int.dita.
Guess you can look at http://public.dhe.ibm.com/software/dw/websphere/PUMA_scenarios.pdf to figure out how you can retrieve user information from the user registry.