If you are with four players, every player must throw an EMP grenade at the same time at a portal. You can construct portals by getting a Denizen on your head, and then walk under an activated lantern. So throw four EMP's at four different portals and you have completed the Easter Egg.
You can also finish the Easter Egg with two players. First, make portals under every lantern post across the map. Next, both players must toss the EMP at the portal, then immediately jump into it and toss your last EMP right at the streetlight you just teleported near. This way, you have thrown four EMP's at four different portals as well, but now with just two players.