From the Labor Pool tab: Click on the 'invite' button on the far right of the Operator's name. The available Ops will then show in a pop-up window; select the Op you want to invite the Operator to and then click Invite.

From the Op: Locate your Op from the Ops tab in your Dashboard or in the Ops tab and click on it. Once on the Op Details page, scroll down to the Suggested Operators section. You can then select multiple Operators by clicking the checkbox on the far right, and then clicking the blue envelope icon.