The WordPress plugin has a couple of functions that can be used to add and remove members from Discourse groups. You will need to write some code, or get someone to write some code for you, in order to use them. There is some information about how to do it here: Managing Discourse group membership with WP Discourse
There are some issues with rate-limiting when these functions are used. They will fail if you are trying to add or remove many users at the same time. I’m going to change the function’s code to solve this problem ( change them to use the sync_sso route instead of making API calls.) The function’s interface won’t change, so it’s safe to try them now.