Table-to-Table: Add All Formula?

Right so you want to create 500 new records in the access group join Table. That middle join Table probably only has two fields. It has a field for related employee and a field for the related access group. So just export the entire employee table into excel, but you only need to export the key of the table which is probably just the Record ID field. 

Look at the record ID in Permissions, which seems to be the list of your various groups and find the record ID for that permission group that needs to have all employees added. Let's say it's record ID 123.

Then just go into Excel and run that 123 number all the way down with the column B of your spreadsheet 

Then import those two columns into the middle table, which is  access groups.