First, you have to set up your own acct. After that, anyone you want to contact also has to have an acct (ie, eBoat card acct). I haven't played with it much, but I believe it works similar to facebook, where you basically send someone a friend request, and you hook up that way. To see someone else's position, they would have to be on your list, and they would have to be updating their position, either by running the app on a tablet, phone, or laptop and updating their position via a built in gps. or manually updating their position on the website.
The programmer of this app sends out a weekly email, if you're subscribed, that has detailed all the directions, as the app has evolved. There is an archive of all these articles somewhere on the Active Captain or eboat website.