To import team members from other organizations, go to the Settings => Team Members page. Click on the Tools dropdown and then click on Import.
Select the Add Existing Team Members tab, and you should see a form like the one below:
Select the team members you would like to import and click the Add Team Members button.
Once added, your team members can switch between organizations.
To jump between organizations, click on the name of the organization in the top left corner and you will see a pop-out with the organizations you belong to.