Hello.............Is their a key that most songs are played in for a LARGE group of singers? In the next few months I will be playing for some chuch groups, retirement homes, and parties. I know individual singers prefer certain key signatures, but is there a general rule for group singing?

Nope, but the basic range of most people will fall WITHIN the staff. Nothing too high, and nothing too low.
Different songs will be in different keys depending on the placement of the notes within the melody, so you can't just assume that ..... "so and so" sings in "G". That's not always the case. Try to keep the highest notes below the top note on the staff ( E or F )and most people will be comfy with that.

Most of the music I play for people is mostly in Cm or Dm in minor keys. There are of course songs in Gm Am Fm... But I play by ear you know, so I don`t know if these key are perfect, sometimes when I play and then singers try to sing and its so high or so low I just switch to another key right away, but I know this other guy he play all of the songs in Fm, and uses Transposer to switch to another key.

Answer--it varies! I've played with church groups, and many of their songs are in E, A, G and D. That's just the simple stuff.....it definitely varies greatly depending on your band leader. Of course some people have limited range when singing, so that will make a difference on what keys they pick.