Tried this out (base install) and will be trying "in play" this weekend
to install the "client" you need "Layar" browser, then install the Battletac "app" on top of that ( just do a search in Layar)
the easy way:
1) go here and get an account:
http://battletac.com/web/guest
a) (button in the top right that says welcome, click the "down arrow" then "click sign in")
b) on the next page click "Create Account" at the bottom
c) do the registration process
2) load this URL on your smart phone and login with the account created
http://lite.battletac.com
the "browser only" version will just give you a map with positions (no augmented reality)
but the "message" functionality DOES work (doesn't work in the "lite" smart phone install version yet)
I tested this with my iPhone and my wife's cheap android phone (LG Eve) and it does work (can see where the other phone is and send messages)