To volunteer at Watkins Glen International you have a few options.
First, if you are a member of the SCCA Glen Region you are welcome to volunteer any club events that are held at the track.
Otherwise, you should consider joining Race Services Inc. (RSI) who provide volunteer marshals the rest of the time. RSI has a number of old campers at their disposal that can be made available for marshals to use during a race weekend. There is a fee associated with joining RSI however it is significantly smaller than that of SCCA.
You may also consider joining the WGI payroll as an employee. It is my understanding that they provide workers for events that are short on flaggers, however your job will more likely involve track maintenance or security services, etc.