Where do Old Order Mennonites find Biblical support for organizing themselves into denominations?

I have a lot of respect for many Mennonite groups' strict adherence to many parts of the New Testament. What passages do Old Order Mennonite groups point to as authority for organizing themselves into large denominations like the Mennonite Church USA, instead of remaining as fully autonomous local groups which share fellowship (but not connecting human power structures) with other local groups? It is possible that I have misunderstood the purpose of said Mennonite denominations; if there is no hierarchical power structure in them, that would render my question moot. But my current understanding is that there are decisions made at the top of these denominations which are expected to be implemented on a local level.