The only thing you know about a person refering him her/himself as an atheist is that this person does not have any beliefs in a supernatural beeing. You know nothing of what she/he believes about anything else.
I AM an atheist, that means i do not believe in anything supernatural. Everything can be explained by naturalistic means. Nothing resides outside our world.
That, however, has no impact on what i think is the meaning of life.
I do not think i have any given purpose in my life. I have only the purpose that i chose;
Beeing happy with my life.
For me, to stay happy, i need to keep the people around me happy. By keeping them happy, im happy. Simple as that.
The things i myself enjoy, besides love and social life ofcourse, are simple things; I develop and i write. That's enough for me.
I even drive elderly people to church every sunday.. im just that nice!
And no deity told me to..