There has been some discussion in the past on whether people became legal members of the WatchTower Society or not when they got baptised and whether this was made clear to them. The WT argues that you are or are not a legal member as it suits them.
Here is a thought...
On their website's privacy statement at http://www.watchtower.org/contact/privacy.htm (linked from the 'request contact' form) they say:
Will the Watchtower Society disclose the information it collects to outside parties?
Watchtower does not give, sell, trade, or rent your personal information to others outside of Watchtower.
So, when they pass your details on to a 'local-yokel' Elder or pioneer to call and see you, that person must be a member of 'WatchTower' or else they are breaking their own privacy statement above (not that breaking promises has ever bothered them before of course).
And what special thing did they do to join? They got baptised, same as you and me.
Just a thought.