Is it proper to give a wedding gift to a coworker if you are not invited?

If you are a good friend and can't make the wedding you should give a little something. But if you weren't invited, this basically gives you no obligation to give. You could have a office gift pool, everyone throws in five bucks or something. Also, check out www.theweddingenvelope.com/calculator.php it is a wedding gift calculator, might help in future. Later, CC