Hi Derek,
Thank you for reaching out to us. I see that you want to add members to a user list in Google Ads API. You should be able to use UserListService to create or update user lists via Google Ads API. For your convenience, you can always check here for service migration between AdWords API and Google Ads API and here for reports migration.
Thank you,
Bryan, Google Ads API Team
Hi All,
That is correct. The counterpart for AdwordsUserListService is UserListService in Google Ads. The fields that you can update for a userlist are listed here. Unfortunately, Google Ads currently does not support adding members to a userlist, but there is a request for this feature to be included. Please keep an eye on our blog for the newest updates and releases for Google Ads. Let me know if you have further questions.