Once you have added a member to a team, you can give them access to a project for collaboration. To do this, click the Invite button available on both the Flow Editor or Mock Editor screens. From the pop up window, click the toggle buttons on the left to a member's name to turn invitation on/off:
Note that turning the invitation on/off on this screen doesn't affect the member's access to the Team.
Here is a quick video on how to invite a team member to your existing project: