Can multiple people put a Lure Module on the same PokeStop at the same time, and does it increase the amount of Pokemon that appear?
You may not place multiple Modules (even of different types!) on a Pokestop as of yet.
You can only place one lure module per pokestop. However if you are in an area with several pokestops in close proximity then you can place one on each to increase the local pokemon spawn rate.