Once you've added an organization, you'll be able to associate people to that organization.
To get started, open the organization's record, and click on the "People" tab.
If you've previously added the person to your contacts, you can simply select them from the drop-down.
If the person does not already exist, you will need to add them by clicking on the "create a new person" link.
Fill in all critical info for your contact, and then click "Save".
*Note: A person can be associated with multiple organizations, and you can add multiple people to an organization.