How can I make sure that only certain people can join my workspace?
If you create a new workspace (button “New project workspace” on the right side of "Workspaces"), the default setting is that you, as moderator, have to manually approve each User ID. Each person who wants to join your workspace (and clicks on the door symbol), must apply to be a member. This application is then forwarded to your email address, and you can then decide whether or not to approve the person via the link in the e-mail.
If approving each membership application to your workspace is too inconvenient for you, you can set up a membership code. This membership code works like a password for users to join your workspace, without you having to take any further steps, see: How do I set up a membership code for my workspace?