In the Settings menu click "Editors"

Click the "Add Editor" button in the upper right corner.

Enter the email address of the person you want to make an editor and choose what kind of permission you want them to have.

Click the green "Save Editor" button in the lower right corner.

Afterward, you will be shown the list of editors you have invited and the status of their invitation. At this point, you have done all you need to and it will be up to your editor to accept the invitation. The editor will receive an email that will allow them to accept the invitation and create a Weebly account if necessary.