FGT is an acronym for Focus Group Test. This is a pre-beta phase in which either Bluehole or En Masse sends invitations out to certain people - much like a closed beta. FGT participants do not test content for errors like a normal beta tester, but rather give their opinions on the already-developed content through surveys and questionnaires.
In order to participate in a FGT, users must be at least 18 years old in order to legally sign a non-disclosure agreement, or NDA, and a confidential disclosure agreement, or CDA. These documents prevent FGT participants from sharing sensitive game and/or gameplay information.