The ability to configure access to a folder is available for professional and enterprise plans.
See "How do I upgrade from a free plan?" if you need to upgrade.
To configure access to a folder, you need first to have "manage" rights to that folder. Account architects have "manage" rights across the entire account and can, additionally, assign "manage" rights to other members of your account.
If you have "manage" rights, then, with the relevant folder selected/open, click on the access control action button (padlock) to open up the access control panel.
On the right-hand side of this panel, you will see the account architects who have full access by default.
Additional access can now be managed via groups (left-hand section of the panel) or individual members (middle section of the panel).
You can add a group or member access permission to the folder by selecting the group or member from the relevant drop down list (using the search field to delimit the dropdown list contents).
A selected group/member is then added to the folder's access control with "view" permission initially. This permission can be, subsequently, changed by clicking on the "Change" link beneath the added group/member avatar. Within the pop-up modal window, you can toggle the access permission to "Edit" or "Manage". Groups/members can be removed from the folder's access control by clicking "Remove" in this window.
It should be noted that groups containing external members can not be assigned "Manage" rights to a folder; only internal members and/or groups containing solely internal members can have "Manage" rights.
In the top-right corner of the access control panel, you can toggle between "Show all permissions" (default), "View permission", "Edit permission' and "Manage permission" to select which permission sets are displayed in the access control panel.