This can be accomplished in the following way: 

1. Create a new 'dummy' or 'test' project to be used for this purpose.
2. After creating the project, open the side menu > Projects > Assignments. Select this new project and assign the employees you want to batch add to this project. 
3. While still on the Assignments screen, click 'Copy Assignments' at the top of the screen. Select the 'dummy' project from the Copy From drop-down. Then select the projects you want to Copy Assignments To. Note: Do not select 'Replace existing assignments when copying,' as this will overwrite any existing assignments in the target projects.
4. When done selecting projects to copy assignments to, click Copy to finish.
5. Delete the 'dummy' project from your Core account.