Question

When address to the staffs of the company, should the "To" line be All staff or All staffs?

Answer 1

also, how can you phrase these sentences better?

We have to cancel this party because we don't have time to organize it this year.

If you are willing to volunteer to help plan the party, you can contact....

Answer 2

If there is more than one staff, you would use To: The Staffs. If there is only one total staff, then it will be To: The Staff.

Question... what party? Xmas? New Years? Be specific.

I would phrase that sentence.
The .... party must be cancelled this year because of a lack of time for organization.

and

If you would enjoy planning helping to plan a party for the staff, please contact ......