Yes, FOLLOWUP is a Scrabble word!

Letter values in Scrabble US: F 4, O 1, L 1, L 1, O 1, W 4, U 1, P 3.

Is it a valid word?

FOLLOWUP is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a piece of work that exploits or builds on earlier work
an activity that continues something that has already begun or that repeats something that has already been done
a subsequent examination of a patient for the purpose of monitoring earlier treatment

Also: follow-up.