What adhesive should I use to make a lighted stained glass mosaic sign?

I want to make a lighted “sign” to hang in our home bar area. I want to make it out of stained glass mosaic peices. What should I use to adhere the stained glass pieces to the glass backer piece and what should I use as the “grout”. I don’t want to use real grout because I think it will be to heavy. Any ideas?