I don't think InGroup will work but I'm sure someone more experienced than me will be able to confirm that.

A way around it though that struck me could be to put the different companies computers in to different OU's and then apply two different policies to the groups.
In the startup script of one policy copy company1.bmp and in the other copy company2.bmp.
Personally I'd call both of the .bmp's the same (or rename afterwards) then set them in the login script as below.

Not a purely kix solution but would expect having the computers in separate OU's would be useful anyway.