Hi there
What kind of access are you talking about? If it's time based access (i.e. the can only access the internet during set times), then these can normally be controlled on the router itself.
If you're talking about site based access (i.e. lock out porn sites), then you'll need another device (or a piece of software running on your server, with the servers IP address as the default getway instead of the router), that will filter out and block content. For an enterprise level company I'd suggest using something like web sense, but there are plenty of free linux based content filters for smaller companies.