Is it possible to make a certain party required for an event?

In my hack, I plan on making two separate places require a full party of certain people: Terra, Shadow, Cyan, and Locke in the Phoenix Cave (yes, Locke), and Strago, Relm, and Shadow within Ebot's Rock.

There's definitely a way to make it so only these people can enter, right? I'm planning huge events around these, and don't want to go too far away without knowing.