User Interface question

Hi everyone, first time poster to these boards. I have no idea if this is the right place to ask this, but here goes:

On the left side of the screen, where the portraits of your team members are, there is a green circle that shows up next to one or more portraits sometimes once the game gets going. What is that supposed to signify? I'm assuming it's relevant, otherwise it wouldn't be there. Can anyone fill me in?

Thank you.