For those interested:
The Slenderman mythos began as an entry to a competition on the website 'SomethingAwful", where images were doctored to show a tall, slender creature abducting children. The idea of "Slenderman" quickly became a meme.
This was expanded by a group called MarbleHornets, who made a series of realistic style videos (think Blair Witch in parts) video blogs (vlogs) and text blogs which told a fictional story about Slenderman but presented it something real. This took off and expanded into several 'branches' of the story, which are all unique but linked. Participation grew, and these various 'branches' of the story were termed ARG's (alternate reality games) and ARE's (alternate reality experiences) -
More info on those.
It could be compared to the Lovecraft Mythos that way, for those who know about that.
The various groups prefer to keep their storylines quite separate, mixing of them is generally frowned on.
One of these spin-off groups is called TribeTwelve, and it's this group I believe these two girls were involved with.
- TribeTwelve is the storyline group (ARG) which introduced the idea of 'proxies', human beings through which Slenderman (he is not called that in this storyline, but it's him) causes harm to people.
- MG's classmate (of the dubious sledgehammer) mentioned MG being in "Slenderman's tribe". As no other segment of the Slendy community uses the word 'tribe', and as 'tribe' is a strange word to use, I'm taking this as another clue that MG was involved with the TribeTwelve group.
"TribeTwelve is thirdly responsible for shaping the mythos of Slender Man,
and chiefly responsible for shaping the modern mythos of proxies and The Collective, of which it uses the most of any series. TribeTwelve depicts the escapades of Noah Maxwell, a college-age resident of a well-to-do Florida community, as he is ruthlessly hounded and stalked by Slender Man and by an extremely cruel and sadistic proxy/collective Member known as the Observer. "
http://theslenderman.wikia.com/wiki/TribeTwelve
http://slendernation.forumotion.com/f6-tribe-twelve