To see the Contacts linked to specific Opportunity select contact option at the right corner. 

1. To add any New Contact to that Opportunity select the add option.  

2. A Contact creation form will appear on the screen. You can easily create the form from there.  

3. If you want to add an exisitng contact to that opportunity then select the ’View All’ option.  

4. It will lead you to a list of existing accounts. You can easily link the intended contact to that opportunity.