Can certain leads be private or visible to only certain users?
Close.io does not currently have a way to "completely hide" or "block access" to certain leads for certain salespeople, aside from having Multiple Organizations.
Close.io is designed to make it possible for all salespeople to see all leads within one organization. We believe this fosters a collaborate sales process, and combined with our non-admin restrictions and activity feed/history on each lead, we believe this can work for most companies.
Note that it is indeed possible to setup Smart Views per-user so that – when using the Smart View – each salesperson only sees "their own" leads. This is helpful for workflow purposes, though it doesn't full prevent access.
Also see Lead Assignment.