Google Latitude, do you have to add it to a phone in order to locate the phone?

My boyfriend and i would like to be able to locate each other but get charged extra for using internet on our phones. exactly how do we set it up to track out cell phones? must you download it onto the phones?Google Latitude, do you have to add it to a phone in order to locate the phone?
yes, you have to download it. it uses the internet so if you get charged extra you might not want to use it.