Short answer: Edit that associate's profile and set the "Invitable" dropdown to "Yes" and then save the change. Once this is done, the associate will appear on the list presented in Scheduling.
Background: When you send out an invitation for a schedule item (such as packing a client), it is typical that not every person in the company needs to receive the invitation. Some associates may only do invoicing or sales. The company owners may not want to receive invitations. Therefore the "Edit Associate" command lets you indicate whether a given associate gets them via the "Invitable" dropdown.