On the people list there is a column called Make Primary. The user information associated with the primary user is currently used on the standard mobile version of your site (if enabled).
We recommend the primary user be your main contact person, as their email and phone number will be accessible on the top of the standard mobile design. Optionally, you can add a general person like firstname.lastname@example.org at the Website Only level, and set up their info. You can then select this person as your primary user.
Only people that have had their staff information filled out will be available to be set as the primary user.