Who are this people in white, often attend to monks? I attended a dhamma talk last month by a Thai monk but English translation were also provided on site.
There was this westerner who is giving English translation but may not look like a full time monk? Although he was in white shirt, partial shaved head, but doesn't seem like wearing a rope. 
Was wondering, how would they be addressed as in monastery? Or they are just layman working for the monastery?

Thanks.
Sadhu sadhu sadhu
He is probably an anagarika.

Anagarika, literally "homeless one", is a stage of practice between lay followers (upasaka) and novice monks (samanera). They usually wear white and act as part time or full time volunteers at monasteries or temples.

They observe the Eight Precepts and are able to handle money unlike novice monks and fully ordained monks (bhikkhu).