Only people whose names are addressed to are invited. There're cases where parents brought kids along for a wedding when only the folks were invited. Imagine the stress you are adding on for the couples scurring to add seats and food. The jury is still out on this one. There are two camps of thought on this one.
Guests usually ask around.
In the Klang Valley it's anything above RM50. Smaller towns ranges from RM30-50. The other camp goes on a 'how close you are with the couples' theory. Obviously, the closer you are with the couple or their families the more you will give. This applies to family and friends.