GLF (or glf) is a common shorthand for "group looking for". GLFM, LFM, GLFP, are sometimes used as well, standing for "(group) looking for more" or "... for members" or "(group) looking for players". They are used by players in the in-game chat to look for more members to join their party, for purposes ranging from quests, missions, alliance battles, and other forms of PvP.
For example, seeing "glf monk" in the in-game chat means that a party is looking for more monks. A more generic "glf more" is a general invitation for any player to join.