You can invite people to your site through the 'People' tab at the top of your screen, so that you can add them to each of your projects:
In the top right of this screen, you'll see two buttons:
The 'Invite Users' button gives you the option to invite people as 'users' to log in to your site, and interact with the projects you add them to. This option gives you a quick way to invite lots of people to your site at the same time, add them all to a particular set of projects, and write a custom message they will all see within their invite email.
You also have the option to add as a user or as a collaborator. The idea behind adding a collaborator is that you can bring your clients into your projects without incurring extra costs. They can perform basic actions such as completing assigned tasks, leave messages and comment on project items. However they can not add new tasks, milestones or files.
Add User or Add Contact
Under the 'Add' button, you can choose whether to add a user or a contact. If you add someone as a contact, they won't be able to log in or have tasks assigned to them - this option allows you to associate them with a project and store their details for reference purposes.
You can use the 'Add User' option here to add a new person individually to your site as a user, add all of their profile details and set their main permissions. You'll have the option to choose whether to send them an invite at this stage, too. After using this option, you'll need to add them to specific projects individually.
Again, when adding a user you have the option to add as a user or collaborator as above.
For more information see: Users and Collaborators