If you have been invited to sign up for something such as for an interview, your Signup invitation(s) will be shown on your Profile page. Each Signup will let you select from the time slots that are still available at the time you are making your selection.
The Freshman Leadership Advisory Council (FLAC) has created a great video that shows how to select a time slot when invited to interview for membership in a Freshman Leadership Organization (FLO). The FLO signup process is identical for responding to any invitation to pick a time.