Under the Contacts heading on the left click the icon
The Add New Contact pop-up will load, on this screen pick the type of contact you are adding to the account then click Add
In the Add Contact panel enter the contacts details (name, company, etc.)
On the Info tab click the Select Info to Add field to select a specific contact detail you wish to add (cell phone, email address, etc.) then click + New
On the pop-up that appears supply the contact information and click Add
Repeat the process of adding contact details until you have the contact information you require
Click on the Address tab
If required/desired populate the contact's address details or select Use Company Address (if the contact's address is exactly the same as the address entered for their company)
Click Save to add the contact to the account