In the event that a Project Manager is in the process of creating a project plan and needs to replace already selected resources or add new ones to the team, they can do this directly from the project plan.
To add a resource to your project team directly from the Gantt Chart view,
Click on the Insert Resource icon
in the tool bar Select the new resource to add from the "Insert new resource" dialog box and click OK.

The new resource will then be added to the list and available for task assignments