Look through the various ERGs to see which one(s) you might be interested in; read their Charters so that you can look at the ERG's mission and goals.
Attend an ERG event as a guest and listen and learn about what that ERG is up to.
Reach out to the ERG's contacts to ask any questions you have.
Once you have decided what group(s) you want to join, talk with your supervisor and fill out the Supervisor Notification of ERG Participation Form found under ERG Forms and Documents | Durham County (dconc.gov).