This is not done in the administration Program. Instead you must log into each company and add the permission.
Details on how can be found here: http://desktophelp.sage.co.uk/sage200/extraonline/Content/ASM/AccountingPeriodUserPermissions.htm
Open the company in SAGE 200
Go To Accounting System Manager -> Accounting Periods -> Maintain Accounting Period User Permissions
Select the user
Then tick the period boxes in the grid ONLY
(NOTE: DO NOT tick 'add / amend / delete periods')
