I think the criterion as it stands is pretty clear. ANY group considering itself Christian is on topic. There don't have to be third-party references - we're not trying to write an encyclopaedia here. If the group exists (possibly, but not necessarily, evidenced by a church website or similar) then that's sufficient to make a question about that group on-topic here.
Obviously spoof questions are not on-topic. A question about the (made up) Christian Church of the Giant Pink Yeti would not be welcome here. But there are many newish church groups around whose existence and theology is not well documented - it would be absolutely wrong to exclude them from this site just because nobody has written a book or a blog post about them.
The exception is when we get into the theological differences between different local churches. Even in the big denominations there are many factions and interpretations, and every local church will have its own customs. We don't want to flood the site with questions about tiny congregations in the back of beyond - so I would say the bar we set shouldn't be about verifiability, but more about relevance. Essentially when we talk about "groups" we don't mean groups of people, we mean groups of groups of people.
Whether or not the question can be answered is a different matter - if nobody in our community knows about that group, then it's unlikely an answer will be quickly forthcoming. But highlighting gaps in our collective knowledge is no bad thing - it actually demonstrates a need that newer users might want to step up to the plate and try to meet.