It depends on the team size and such, but generally they assign each team two rides where they are required to provide a number of volunteers. The number is based on how many riders the team has signed up. The volunteers don't have to be riders, they can be friends, family, coworkers, etc. The duties are usually either running the sign-in table, working one of the breakpoints, or driving a sag vehicle. My preference if I can get it is to work the sign-in table, because you can get your bike all ready and as soon as the big crush of sign-in is over, you can jump on your bike and still ride. Otherwise, if you are driving sag or working a break point, you will miss out on riding that week. If you have a well run team with people who will step up and do their part, generally you will only have to either be a volunteer or provide a volunteer in your stead for one ride out of the series, so it isn't a big deal to miss one out of 11 or 12 rides.