You can get this one of two ways. Create your own syndicate and invite 4 or more members to it or get invited to one that has at least 4 members already in it.
To create your own syndicate, from the main menu, select
Co-op.
'
' over to '
Syndicate' and select '
Create. Customize your syndicate settings to your liking.
To invite members to your syndicate. Enter a lobby and highlight a gamertag and press the '
' to
invite that person into your syndicate. You can send invites to people who are already part of a syndicate, but they'll have to leave their current syndicate in order to join yours. It's best to invite people who aren't part of one yet.
If you have a group of 4 friends, you could create a private lobby and send the invites from there.
Once you have 4 members in your syndicate, the achievement will unlock.
Good luck.