Opinion!

say me and two other people see the flop, does anyoen know what the odds are that one of the other two people has flopped two pair or better?

How about 4 people? 5? 6? 7? 8? 9? 10?

I'm trying to determine the EV of going all in after the flop if you have an overpair..

depends on the flop and the kinds of cards they play

say people like to play JT type of hands and dislike playing J2
then a JT7 flop is much better than a J72 flop for making two pair hands

wtf? No way OP is actually doing proper analysis if he's asking this question.

Simple (i.e. inexact)

5% to flop 2 pair or better, thus 95% that you don't

2 people, 0.95^2 = 90.3% fail, or 9.7% hit
3 people, 0.95^3 = 85.7% fail, or 14.3% hit
etc

Don't know if this will help, but this is a good place to start. Has all kinds of odds charts and stuff.

http://www.flopturnriver.com/poker-odds-charts.html