Hi, this is a 12-member team, we focus on building android LBS application. We already released 2 LBS applications(one is pulished on Android Market).
According to your requirement, we can create a client/server application, the client is the android application, which can get location from GPS module periodically and post to the server. And the server is a PHP module, which can store the latest location of client to database and check if the client is in the defined scope. If the client is out of the defned scope, one message will be created and then a monitor user can get an alert from the web site.
BTW:
1. the defined scope is not editable, which is a quadrilateral area in google map
2. the members are defined in a confiugration file with plaintext
3. the account of "super user" to monitor other members is also defined in the confiuration file with plaintext