It's all kind of moot since they are all complicit in the deed, regardless of how it was done in the show. We may get the actual details later. I wouldn't expect someone to look down in the cup and drop something into it from a few inches above, though. Since Margaery is closest to the king, she would be in an ideal position to poison Joffrey. Olenna is not - There's a good chance she would never have the opportunity. As I mentioned above, as well, when Olenna leaves the table and walks back towards her seat you see a movement which could be described as a handoff of some type. For Olenna to poison the cup she would have to reach over the table while the pie cutting was going on and drop it in - a much more obvious maneuver (and again assumes that she knew she'd have an opportunity like that to begin with).
Again, I'm just speculating on how the show went with it, I definitely could be wrong. But I present Exhibit B:
(this shot had no purpose other than to show Olenna walking back to her seat)
It's very quick and subtle, but margaery can be seen making a movement towards Olenna as she passes after speaking with Sansa - a movement towards Olenna and her hands look as if they go up and down quickly. (If you get a chance watch the scene in HD and the movement is much more obvious.
While not directly stated, Margaery definitely knew about the plot, whether or not she did the deed. You would think both Olenna and Margaery would have thought it through a bit before doing anything, so the fact that the Tyrell's are like oh shit, we didn't think this through really doesn't prove anything.
For reasons of the show, it definitely makes sense to tie them together since the was between Margaery and Cersei is going to get heated up this season, not Cersei and Olenna, And MArgaery is about to be painted as a bitch in her own right.
So yeah, until the show explains otherwise, I'm going with Olenna grabs poison from Sansa, who was the unknowing middlewoman of LF and Olenna, and then hands it off to Margaery since she's in the best position to poison him, which she does while the cup is in her hands. In the show, she is the only person aside from Tyrion, Joffrey, and Sansa to touch the goblet. In the book, Joffrey places the cup down on the table himself.