If you are trying to give someone else the use of your elite benefits or trying to get points from someone else's stays there are no guarantees. You can try to add their name to a reservation in your name, but it is possible the hotel won't offer your elite benefits or any points until/unless you check-in. Personally, I'd be very careful of trying this out too many times. Even if it works once or twice, it might be possible for Marriott to notice the pattern and then they might take some type of action that you wouldn't like against your account. To me, the loss of my account would be far worse than a few extra points or a friend getting a breakfast for a few trips. YMMV.

If you are in a situation where you made a reservation for yourself, your plans have changed and there is someone else that would like to have the 'no longer available' rate you received, I have been able to update a reservation name before, but doing so would take your name and MR# completely off the reservation (as if it were originally made in their name and not your's.)

I did that for a conference trip a few years ago. I was planning on going to a conference in New Orleans and was able to snag one of the "tiny" rooms at the Ren Pere Marquette Hotel for about $100/night (where everything else was over $250/night.) My plans changed and I couldn't go, but a friend was still going. So I called Marriott and we were able to transfer the reservation over to his name. So that can be done.