To bulk assign access to employees, you need to create an insight report first, which you select and filter to include the employees you want to give access to.
The mandatory column field you need in the report is Person Reference.
After the report is created, go to Data Centre - View All and search for Bulk Assign Access.
Then you can select the report you have created, click Apply.
A list of the employees from that report will show up, there you can also manage if you want, select/unselect some from the list.
Click assign access, and in the next screen you will be able to manage what access you want to add/remove by clicking the corresponding buttons.