To assign a contact to a company, or change the company a contact is associated with, go to that person's contact card and click on Edit.
If the contact is not assigned to a company, the Company field will say Company in grey, and if they are connected to a business, the Company field will list the name of the company. To assign or re-assign the company, start typing the company name. Existing companies will pop up as you type their name.
Once you have finished making changes click Save.
You can view the contacts belonging to a business in the bottom-right hand corner of the company's card (you might need to scroll down).
To add employees to a company, just click the "+" icon in the Employee box. As you start typing, suggested contact names will appear.