Summary conclusions:

The 527 group known as Swift Boat Veterans for Truth would have you believe that they represent a significant portion of those who served with John Kerry in Vietnam. In fact, the data shows that they are only a tiny group and that they do not have any special insight into John Kerry's record of service in Vietnam.

SBVT's claim that they represent "more than 250 Swift Boat veterans who served with Senator John Kerry in Vietnam" is untrue. Only about 28% of the SBVT's signatories can truthfully say that they "served with John Kerry in Vietnam.
SBVT's signatories are only 6% of the total number of those who served under Swift boat commands in Vietnam.
SBVT most likely used a simple email campaign to solicit their membership. The data suggests that there are likely hundreds of Swift boat veterans who received email solicitations from SBVT but declined to join the group.