My first idea, was for everyone who got 4 members by April 2nd to be invited to like a Sand party place.
Also, I was thinking that in the long run of the Ultimatum, whoever recruited the most people to the site would become Kazekage for a day or something.
And of course anothe idea was to make Chuunin/Genin Squads to get on it and they'd do missions together.