To manage collaborators, first navigate to your team or project first.

Once there, you'll have the ability to add additional collaborators at the top right of the team view.

Each project maintains it's own project team and you can manage them individually by clicking the button next to the list of current collaborators.  This will open up an edit panel which gives you access to manage existing collaborators and send invitations. 

When sending new invitations, you can select the type of role you wish that user to have once they accept it.  Members have full access to the project.  Guests have view only access.  When you click "Send Invitation", the individual will receive an email with details on how to accept it.  You can re-send that invitation at any time.

Once the invitation has been accepted, you'll see them show up in the list of collaborators and can manage their status from there.