I'm in the west/midwest US. I almost always hear Meg-an rhyming with peg, but I wouldn't necessarily consider May-gan to be a different pronunciation, just someone saying the vowel extra-long. I wouldn't say May-gan unless someone specifically told me that's what their name was (although it also depends on the spelling. If there's an extra letter like
Maegan,
Meagan, etc. I'd be more likely to say May-gan).