Yes, REGISTRAR is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, G 2, I 1, S 1, T 1, R 1, A 1, R 1.

Is it a valid word?

REGISTRAR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a person employed to keep a record of the owners of stocks and bonds issued by the company
the administrator responsible for student records
someone responsible for keeping records

Also: record-keeper.