To add/remove users from Matter, simply add or remove them from the Slack channel.
Our Web App syncs with Slack every 24 hours. During this sync, we add/remove channel members to the Web App workspace.
If you add someone to the channel and you don't see them in the Web App, you'll need to wait 24 hours for the API sync.
Note: You may need to be a Slack Workspace Owner/Admin to add or remove users in a Slack channel.