Maybe some sort of “military broadcast” or other hints about a military base of some sort? They should know that others may try and clean it out before they get there which introduces a ticking clock element. Maybe the message includes a warning that if the security system doesn’t get disabled the base will self destruct in X many hours?

Then when they show up, they encounter enemies (zombie mob/other survivors) and have to work together to defeat them. When they succeed, they get the things they need (food, weapons, a defendable base)

Pretty much anything that telegraphs people gathering somewhere with supplies, or hints towards somewhere that can be fortified (army base, buildings that can lock down like government buildings, schools, possibly hospitals, etc) is a pretty easy way to get your players to gravitate towards it.