Does the presence of high level trainers have an effect on low level ones? [duplicate]

There really isn't enough data to say either way yet.

Personally I think higher levels do attract a better quality pokemon to the area (thus anyone can catch it-but you need the high level to trigger it's spawn.)

Just personal experience but could be completely RNG

From my own experience and that of people I know, your level affects the CP value of Pokemon you encounter, not necessarily the Pokemon themselves. So, if one of you is level 15 and the other is level 5, you will both encounter the same Pokemon in the same locations, but the level 15 might encounter a CP 500 while the level 5 might encounter a CP 50.