To add or remove members in your workspace, log in and go to Members in the left side menu.

Type in the person's email address and click on Add Member. Now, a message goes to the person you've added that says they've been invited to a workspace.

Please note that maps in your workspace will not automatically be shared with the person you add as a member. Sharing maps is separately done. Here is an article on how to share maps

Click on the garbage can to remove a person from the workspace:

Note that any maps shared with the person will still be shared with them, but only with view access when you have removed them from the workspace.

Guests on maps in your workspace, can be found in the left side menu under Guests:

If a former member has been owner on a map, those maps will be transferred to the workspace owner when you remove the member from the workspace.