Have your colleague create a CircuitHub account. Take note of their user ID (please be mindful it is case sensitive).
Go to your Circuithub home page. On the right hand side will be a column titled "Your Organizations". Click on the organization in mind, and a new window will appear.
In this new window there will be a small box with 4 tab options, the bottom tab will be "Members".
On the "Members" tab a list of the users in the organization will appear in addition to a text bar at the top of the list. Type in the user ID of the colleague you wish to add (please be mindful that this is case sensitive), and click the blue icon to the right of the text bar when finished typing.
For more information on project collaboration see: http://docs.circuithub.com/articles/62933-how-can-i-share-a-private-project