Perpetua Technologies has a low-cost tool called Samariteam that a number of CERT and VIPS sites are using. This system is designed specifically for Citizen Corps programs and works really well. It's designed from the ground up to be hierarchical and reduce or eliminate the burden on volunteer leadership to keep track of contact and location information.
You can learn more about Samariteam at www.samariteam.com or feel free to let me know if more information is needed!
CERT and MRC volunteer