Just go to a portfolio you want to add members to.
Click on 'Edit portfolio' on the top right.
Click on 'Members' on the top right to add project collaborators.
If you add a member, they can edit the portfolio content but not change the project settings. Members will use up their individual uploads while working on a project. A GURU account user will have an unlimited number of uploads as a member so get one to help you out! :)
You cannot add members to your main portfolio associated with your account so make a different portfolio that you wish to add members to. Or you could make another portfolio and turn that into your main one so you can add members to your current portfolio.