Here you will get an introduction to the various user levels.
Intervals includes four user levels with project level permissions to control what people see and what they can do. The user level (Administrator, Manager, Resource, and Executive) controls the general view for that person. The project permissions control which projects a person can work on.
Administrator level users are unique and have access to all projects but the other user levels only see projects where they are a member of the project team. If they do not have access to a project they do not know it exists.
Administrator level users see all other users, but non-administrators are bound by the project permissions. For managers and resource level users they must share at least one common project to see each other. If the two users do not share access to at least one project they cannot see one another.
It might be helpful to create a few sample users and login as them to see what they see. This can help make sure you set the desired view for the given person. Here is a help article on creating people, a help article overview of the user levels, and another on how the project level permissions work.
Watching Intervals in action is often the best way to harness its potential for your business. These videos explain creating users and the four user levels in Intervals.