The_________are meeting at the White House.
I can't figure out how to use Chief of Staff in the above sentence.
Chiefs of Staff
http://www.jcs.mil/
If the Chief of Staff is meeting with someone (it says "are", so he's not meeting with himself), who might he meet with?
Yes, each branch of the military has a Chief of Staff. The President has a Chief of Staff, too.
http://grammar.ccc.commnet.edu/grammar/plurals.htm
Scroll down and study how to form plurals of compound nouns.
To incorporate the term "Chief of Staff" into the given sentence, you can modify it as follows:
"The Chief of Staff and other officials are meeting at the White House."
The term "Chief of Staff" is used to refer to the highest-ranking staff member in a hierarchical organization, especially in government or military. By appending it to the sentence, you are specifying that it is the Chief of Staff who is among the individuals gathering at the White House.