Since the Workload Space gives you a better grasp on who’s on the bench - or who has capacity to fill - this option allows you to quickly place them on projects that fit their roles.
To view unassigned Work Plans, click three dot menu next to the Member name and select "View Unassigned Work Plans".

A split screen will show and display all Work Plans currently unassigned, but that have the same Role as the Member’s selected Default Role. To assign the Work Plan to the Member, either click to open the Work Plan to modify the assignee - or drag and drop the plan to the Members row.
Click the X next to the Member, or the close button to close this split view.

