(1) It is known that 720 sets of single tables and chairs are needed. If it takes 6 days to complete this production task:

Production of 720÷6= 120 sets of single tables and chairs every day;

(2) If the production task is required to be completed at least 1 day in advance, then:

At least 720÷5= 144 sets of single tables and chairs should be produced every day.

Because the factory must produce tables in groups of 5 people, each group can produce 12 tables every day; Chairs must be produced in groups of four people, and each group can produce 24 chairs every day, and the total number of employees will increase to 84.

Therefore, X groups of production chairs and 2x groups of production tables can be provided, including:

5*2x+4x=84

It is easy to solve x=6.

Therefore, the employees who produce tables are: 5 * 2 * 6 = 60; There are 4*6=24 people producing chairs.