This can be accomplished in the following way:
- Create a new 'dummy' or 'test' project to be used for this purpose.
- 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.
- 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.
- When done selecting projects to copy assignments to, click Copy to finish.
- Delete the 'dummy' project from your Core account.