In one Snapcraft Clinic session, I asked about gaining write access to one of the snapcrafters-adopted repositories (snapcrafters/nano to be specific) and got a reply that only the fellows in Canonical are qualified for such access (probably more because that snap is under classic confinement, though).
For collaborative maintenance, I previously proposed Snap Dump: An experiment of maintaining snaps, together, as an organization which is meant to gather people interested to co-maintain a pool of snaps to improve snap quality / host snaps that are not high profile enough to be adopted by the Snapcrafters organization. Unfortunately there isn’t any activity since then.