Outsomnia
Momentum was founded by Jon Lansman after the defeat of Ed Miliband as an organisation to gather together members that would support a broadly-based Leftist direction within Labour.
Initially, it was not a part of the Labour Party - although it now is affiliated to the Labour Party and it's members are now Labour Party members.
Lansman was helped in the establishment of Momentum by [James Schneider??? I forget] and consequently a large part of the initial membership was drawn from pre-existing groups out with traditional Parties such as the Stop the Cuts organisation.
Members were also drawn from the Green Party and climate activism, from people who had left the Labour Party over the Iraq War, existing Labour Party members, and other groups.
Whilst some may well be from groups that were banned from Labour Party membership, many were and are not.
The group is a broadly left coalition, with a strong emphasis on education, outreach and activism.
It's not a homologous mass - for example, it's quite split in the issue of Brexit.
Is it a Corbyn cult? Well, I guess we'll see if/when Corbyn steps down.
Certainly, Lansman hopes it will continue as an organisation providing support for a Leftist agenda within the Labour Party.