Here is the link to sign up for the July 14 - 15 weekend. Looks like you will have to buy each day since they do not have a weekend bundle link. If you are a member, sign in and then buy your day(s) as it is will then show the member's price.
http://www.sportbiketracktime.com/buy-track-time/Scroll down to July, expand the list by clicking the blue arrow and choose the July 14 and 15 dates.