A group of select people.
One is classified as team X material when they have completed something badass or extreme.
When one does something worthy a pre-existing team X member will induct them by saying the simple phrase "team X" and the official sign of an x made by the forearms of the inducted and inductee will be made.
When inducted one is an example of pure badassness, therefore induction of new members should not be earned by unworthy actions or for no reason.
"Nice buzzer beater man, we got the championship" "Team X"
"Man what is that?"
"A title for all the worthy badasses"