As an Admin or Billing Moderator, you have the authority to determine the process for accepting new users into your Workspace.
Go to your Workspace's Settings page, then click Security.
Under Workspace's visibility settings, you may choose from the following.
Invite only - new members must be invited.
Visible and allow anyone with certain domain to join - members that share the same domain listed under the nominated Domain list can see the Workspace and join without Admin approval.
Visible and need administrator's approval to join - members that share the same domain listed under the nominated Domain list can see the Workspace and join but have to wait for Admin approval.