You can manage channel members via the "Manage members" dialog.
You land in channel members dialog via the channel context menu in the tree. For this, hover over your channel in the channel tree, you'll see the context menu button (three vertical dots):
Click on it and you'll see the channel context menu. Choose "Manage members":
Once you click on the "Manage members" menu item, you'll see the popup where you can manage the channel's users:
Note, that you can only manage users that are already part of your workspace. If you need to add users to the workspace, please read this article: