This explanation you give would explain a group of, say 20-30, but definitely not hundreds. These "Mega" groups take away the security of smaller parties trying to do the same thing. They "Market" their huge groups with multiple spam posts and by glorifying the size of the of the party as if it matters that they have several hundred members. Just a thought coming from a member of a smaller group.
https://www.visible.com/p/SanVisible
