From the Sync function you can add any document to a channel using channel() then give the user access to that channel with access().
Here’s an example that assumes your document has the property “username” this assigns the document to the channel “channel.username” then gives the user username access to that channel.
var username = doc.username
channel(“channel.” + username);
access(username,“channel.” + username)
They have some good tutorials with full sample projects here: Developer Portal | Couchbase