For your case, write a PL/SQL function(takes person and/or assignment id) which does whatever you want and return the value.
In Fast Formula, get the value by calling the above created PL/SQL function(by creating fast formula function)
Same way, call this PL/SQL function from anywhere(may be from Fusion as well. Sorry I am not well versed with Fusion)
In addition to peddi, if you were looking at how to get the reportees for a respective manager then please find the query below:
SELECT DISTINCT papf1.full_name leve1_full_name
, papf1.employee_number level1_empno
, papf2.full_name leve2_full_name
, papf2.employee_number level2_empno
FROM hr.per_all_people_f papf1
, hr.per_all_assignments_f paaf1
, hr.per_all_assignments_f paaf2
, hr.per_all_people_f papf2
WHERE papf1.person_id = paaf1.person_id
and papf1.current_employee_flag = 'Y'
AND paaf1.supervisor_id = papf2.person_id(+)
AND papf2.person_id = paaf2.person_id
and paaf1.supervisor_id = 500
and trunc(sysdate) between papf1.effective_start_date and papf1.effective_end_date
ORDER BY leve1_full_name;
Once you have the list build a function which will execute the business logic.
As Peddi suggested, you should be able to write a Pl/sql function and get whatever value you need into the Formula.
Also, try using the Fusion forums for this -