Just click here, or go to 'Contacts' on the left-hand side of the Dashboard.
You'll see a list of all your contacts on the right side of your screen. Select the contacts that you would like to add to a group and click on 'Add to Group'.
Choose the group you would like to add them to from the drop-down menu and hit ‘Add to Group’.
Vice versa, use the button 'Remove from Group' to delete contacts from a group. For more information on removing contacts, read one here.