Me and my friends (Goldheir and Dawnglory) want to combine our diamonds we won from the peak a door to buy a mystical essence. Is it possible for them to give me their diamonds so we can create our dream pet?

Yes.

If you were to give your friend diamonds, your friend would go onto the Exchange and set up a trade to sell 1 gold in exchange for the number of diamonds you hold. You would then find that trade and "Purchase" effectively giving the number of diamonds in exchange for 1 gold.

Warning: You need to make sure you get the trades the right way around - if you were to set up a trade to sell the number of diamonds for 1 gold, the trade would be snapped up as effectively anyone could grab your diamonds for next to nothing - you don't want to do this. Basically, you want to set up the trade so its generally undesirable to an outsider and you would be getting the significantly better deal (e.g. effectively free diamonds) if some random was to "Purchase" before the other party to the planned trade had the chance.

Edit: For typos

Thank you!