Note: If you received an email that users are waiting to be approved, but they are not listed in the User's list of the space, then most likely another Administrator has either approved or denied the user.   All users with Administrator permission on a space can add or remove users at any time from that space.

In the workspace security settings, one of the options available for Administrators is to enable or disable outside users to request access to your space.  If you are an Administrator, check this box to enable Request access, or uncheck it to disable it.

When any user who is not logged in reaches the URL for your space, a Request Access button is shown to them, if this setting is enabled.  If the setting on the workspace is disabled, then no button is shown.  Click on this button to request access to the space.

Users then are prompted to fill in the details of their request and solve a small captcha to confirm the request is authentic.

Finally, all users with Administrator permission on the space are sent an identical email with the details of the request.  The Administrators are then directed to the user's list to either accept or reject the request.  If there are users waiting to be accepted in the user's list, click the "Request Access" button on top of the user's list to review the users waiting for approval, and then click to either approve or deny access.