The achievement is still unlockable, even after the closing of GameSpy's servers.

To unlock the achievement you'll need to be in a game lobby (the screen before you are in first person). So if you are in control of your character and then invite someone the achievement will not unlock. The achievement will only unlock if they are in the lobby.

This is what the lobby screen looks like:

There is a 12 month time limit for people to share the achievement with others.

If you need help unlocking the achievement here's a list of users who are currently able or willing to help:
(If you would like to be put on the list, just leave a comment saying so or send me a DM. If you want off the list please do the same, no explanations required)
Eimnesia