In addition to managing user access via Roles (see this article for more info) you can now assign Groups to Users to control which Forms / Devices they have access to.
Managing the User Groups is done from the same edit page as the Users Role.
First, after logging in to your account, navigate to your dashboard. On the left side of the page you'll see a "Settings" button. Click that button and then click the "Manage Users and Roles" link that appears underneath. You'll then be brought to a separate page.
To alter the user, click the pencil icon on the right hand side.
Then, navigate to the "Groups" section and tick/un-tick the groups.
Click "Update User" and you're done!