By design the system does not add new Group members to already-in-motion Group Items, allowing you to choose what they should be a part of. This avoids potential messes that could occur if you added a client to long-lived Groups that have many past assignments, such as well-in-progress Metrics.

To add a new group member to an existing group item, find that item in the Group Stream and click the edit icon found in the lower right. 

From there, update the item to include the newly-added Group member and you're all set.