I can't think of many acronyms that apply to GRO (Ghost Recon Online).
CB: closed beta
OB: open beta
DPS: damage per second
AOE: area of effect (like with grenades)
cap: capture (like with capturing points)
bubble: Aegis shield (the specialist ability)
GM: game master (like an admin)
regen: health regeneration
FOTM: flavor of the month (temporarily OP builds that lots of people take advantage of at the same time)
EMP: electromagnetic pulse (the specialist's Blackout ability)
AR: Assault Rifle, a primary weapon for the Assault class.
LMG: Light Machine Gun, a primary weapon for the Specialist class.
RP: Requisition Points, the main currency used in Ghost Recon Online.
SMG: Submachine Gun, a primary weapon for the Recon class.
Camp: Staying in a single position for an extended period of time. "Sniper camping behind the dumpster."
Flank: The side or back of an object. If you're being "flanked," that means the enemies are coming at you from the sides or behind you. "Watch your flank."
Nade: Short for "grenade." "Nice nade."
Ping: To detect someone with the Recon ability Oracle. Named after the pulse of sound emitted by sonar equipment, since the ability sounds similar. "I just got pinged; I need to move."
Spawn (verb): A character entering the game, either for the first time, or after death (the latter called "respawning"). "I'll spawn in three seconds."
Spawn (noun): The area of the map where a character enters the game. "Make sure our spawn is clear."
Tango: An enemy. Haven't heard this used too much in Ghost Recon Online. "Tangos coming right to you!"
NOTE: I'll be editing this post with more definitions, instead of adding new posts.
Cloak: a recon's ability to go "invisible": "We have a cloak behind B"
Push: to aggressively move forwards, or to attempt to dislodge a stubborn enemy from his position: "Push C", "I'll push this sniper, you cover me"
Support: normally fire support, can also mean bodies (players) in the cap point: "Support my push (I need suppressing fire so I can attack this guy safely)", "Need support at D (can mean: 1: I need fire support, or, 2) I need more bodies in the capture area, as more players in the cap zone = faster capture)"
Block: to delay or stop an opposing team's Push: "I'll block this hallway, you guys flank"
Don't forget NME
which is short for 'Enemy.' Though, it isn't used in this game at all xD
(Something I learned in Project Reality Mod)
Another I learned from that old mod is 'RTB' which means 'Return to Base'