Seagulling is a variation on the sexual practice of bukkake, with the crucial difference that in this case the recipients are unwilling.
One person learns about two people that are going to have sex and works out the location of the aforementioned event. Then he gathers as many guys as possible and packs them into the room where it's going to take place. The guys hide in cupboards, behind curtains - basically, wherever there is a place to hide. Then when the couple look like they're about to finish, everyone runs out and ejaculates all over them and then proceeds to run around them, flapping their arms like birds and making seagull noises.
Mike's friends thought seagulling on him and Susan would be great fun, so they all hid until the time was right.