If the Pell grant was awarded to you by the school, then the grantor ID is the Filer's ID shown on he 1098-T. It is common for a school to reward high performing students with a "free" class or two for a semester or more. They'll do this by awarding that "free" class as a grant. (it's a tax deduction for the school)
If you applied for the Pell grant with a third party (this is also common) and the Pell grant was paid to the school by that third party, then the grantor ID will be on the approval letter or other paperwork that came with that letter, from the authority that approved and paid the grant.