If a section has the access level of "Private", then the only users who will be able to access the section are the users who are in the section's User Access list or who belong to a group in the Group Access list. In order to allow additional users to access the section, they must be added to the User Access list or Group Access list by the section owner.

Make sure you are already logged in. Navigate to the section in question, then click the "Owner" link in the Tool Bar In the page that appears, click the "Group Access" link under the "Sections" heading. Figure 1 shows the Group Access screen.

Group Access List
Figure 1: Group Access List

Simply check off any groups that should have access to the section or uncheck groups that should not longer have access, then click the Update button.