Yes, as per the title of the thread, the registration for this event starting next Friday the 11th is closed.
I believe you can still sign a squad up for FSO, but your squad will have to wait till the next event in May to participate as a squad.
I don't know how many people you have to fly, but you may be able to find room for them with another squad if you post in this BB asking for a place. You will not be flying as your squad but you would be able to fly.