If you have already opened a survey, you can still send it to additional people:
First make sure you have added your new people to your account (click here for more info)
Go to the Surveys menu.
Click Settings (the clog icon) for your chosen survey. This will take you to the settings of your survey.
Tick the box next to the names of all additional people you would like to send the survey to.
Click Send Survey. This will only send the survey to the new people added, and will not re-sent to those who are already included.