all the players need to buy the DLC for 15$, but the admin of servers won't need to buy it.
''Admins:
All servers will have First Strike available on March 25th. It will be at the server admin’s discretion whether or not to run DLC maps on their server. Admins will also have the freedom to choose which First Strike maps will run on their server.
Note: Admins will NOT have to purchase a copy of the map pack in order to install it on their dedicated server.''