Mohammad Daoud has a great look at setting up Account Level Security. The only addition I would make is to turn on the Account Level Security checkbox last if you are doing this in a production environment. If you don’t, the chart of accounts disappears for all user not yet setup with Account Level Security. It’s rough on the career.
Account Level Security Setup
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.