To volunteer as a marshal for a United SportsCar event there may be a specific role that could get you accepted to any IMSA race they put on. As far as I know however, to be a flag or track marshal, each event is responsible for their own marshals. Since the majority of TUSC races take place in the US or Canada, few clubs handle all marshal recruitment. Often TUSC run as a support series to another event like IndyCar so reaching out to their organizers may be appropriate.
So how do you go about registering?
There are a few options however you will have to do some research. The most obvious option is to contact the event organizer. Typically, events like Long Beach GP publish a web site: gplb.com The web site is frequently advertised by local news outlets and anyone can theoretically register. There is obviously no guarantee of being accepted. Be careful not to sign up for a volunteering role that doesn’t involve marshaling. For example a lot of volunteers are used for everything from crowd control/security to garbage collection.
The other option is to become a member of a marshaling club or organization. For example in the US being a part of USAC or SCCA allows you to get the necessary training to qualify as a pro-event marshal. So that when registration opens, and a particular organization is selected to provide marshals for that event, you will be more likely to be accepted as a member of that club.
The final option is to contact the circuit where the event takes place. For example when I wanted to marshal MotoGP, the only contacts I could find were employed by Indianapolis Motor Speedway. So it was an obvious choice of first contact (this was before I learned of USAC). You may go as far as asking for a job with the circuit so that when the big event comes around you have the possibility of working there as an employee. This is the case with many New Jersey Motorsport Park marshals who were specifically employed by the track to work various track rental days, and who were also given an opportunity to work major events when the majority of marshals there are volunteers. Working for the track is not practical if you only want to volunteer weekends. Similarly, some street circuits are only assembled for a particular event.
Best of luck and if you have pertinent information to share, please leave a comment below!