Visible to only some people on Whatsapp? I am afraid of using whatsapp just to avoid so many unwanted messages or unnecessary people contacting you. Just wondering, is it possible that am visible on whatsapp to ONLY some people (all my contacts shouldn't come to know)?
Yes, you can choose the option to be visible only to your contacts.  

Go to Settings --> Account --> Privacy --> Who can see my personal info and choose Last seen, Profile photo, status to My contacts. But configuring this, everyone who is not in your address list can know that you have a WhatsApp account, but they cannot see your last seen, profile photo, status.  You can also block unwanted contacts here. By blocking a particular contact, the other side will not see your last seen, profile photo, status and additionally you will not receive any messages / calls from blocked contact, messages will be shown as undelivered to them.  

When you are in a group, provided you set the correct privacy options, the group members can only see your number and the name that you used while adding account on the device. There is no way to change this behavior.  

Also, you cannot avoid unknown users adding you to a group. WhatsApp doesn't ask you whether you wanted to join a group or not. All you can do is to quit after joining and block the ones that joined you. Blocked contacts couldn't add you to a group.