My guess: If you take the time to temporarily invite them to your fireteam you could. Otherwise, my guess is you would do a victory dance and go on about your business. From what I remember, Bungie said you can only speak directly to people within your fireteam.

I'd like to see Bungie add a community chat option for just such an occasion. If and when a community event happens, it would be really cool if a "phantom" halo was implemented around the event, that way all Guardians within that halo can communicate with each other. Even after the event ends, to kind of hang out water cooler style. And once the fireteams broke off and left the Halo, it would dissappear and you'd go back to normal chat.