When I'm forming my posse I want to use that ability
Can't use the shootout abilities until AFTER the posses are formed. So you would form a posse normally, and then (if you still had control), you could use the deed to bring in another dude while laughing evilly.
The way I've been playing it (not that that is binding), your opponent, in bringing someone else to that deed to tip the influence, would have control of the deed. You would be denied the ability to use it.