Getting person id from userId:

select papf.PERSON_ID,papf.LAST_NAME
from fnd_user fu
,per_all_people_f papf
where fu.USER_ID=:usid
and fu.EMPLOYEE_ID=papf.PERSON_ID
and trunc(sysdate) between trunc(papf.EFFECTIVE_START_DATE) and trunc(papf.EFFECTIVE_END_DATE)