To add Team Members into the current project, select a task and go to menu Team – Assign –  Team Member. It will open the Assign Team Members to This Task dialog, which lets you select who do you want to add to the current project. Once you click Assign, all the selected Team Members are added as Work Resources into the current project.  

You can also add team members from Gantt Chart view, using the Resource Names column.

When you open the Resource Names drop-down menu, you will get a list of all resources in your project and the Team Members (marked with an icon). Note that all the Team Members are visible in this list (added or not). Selecting any Team Member in this list will add it to the current project (only once). If you want to add a new Team Member, click Add Team Member button. 

Another option to add team members in your project is to use the Resource Sheet view. 

The Resource Name editing control provides a list of not added Team Members and the “Add Team Member” button. If there are no Team Members to suggest, the button alone is displayed.