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There are 2 main types of grout: Cementitious and Epoxy. Each comes in sanded, for use in cracks wider than 1/8 inch, and unsanded, for narrower cracks. Sanded grout is stronger than unsanded. There is some concern among tile installers that sanded grout will scratch the surface of glass tiles. I use my hands rather than machinery for polishing and have had no problems on stained glass applique.

Epoxy grout is stain resistant and I recommend it on indoor tilework. Glass is another matter: epoxy's different coefficient of expansion and general stiffness often results in pressure fractures or scallops along the edges of glass pieces. These appear several months down the line so even if the piece looks good when you sell it, it may fracture later. I don't use it for this reason.

Cementitious grout needs to be polymer modified to add a bit more flexibility and frost resistance. I used to use Custom brand from Home Depot but they recently (mid 2005) modified their formula and I have been getting pressure fractures and scallops along the edges of my glass pieces. I have since switched to Laticrete brand.

Laticrete Sanded Grout (#1500) is usually sold in commercial tile stores (contact me if you have trouble getting a little bit to do a table). You need to use the admixture rather than just water. The formula I use is 1 cup dry grout to 2 oz. admixture (#1776). This covers a 20" round table. If your table is larger, mix another batch after applying the first rather than mixing a larger batch. Apply the second batch to both sides of the first batch and so on so that the next batch contacts the most recent batch, not batch one.

Choose your grout color with care. Dark grout makes the dark pieces merge and accentuates the lines between the light pieces. Using a light colored grout reverses the effect. While many artists use colored grout, I usually prefer medium gray grout in my designs. Adding colored grout seems to me to detract from the glass.

Sealer: Cementitious grout also needs to be sealed after it cures to make it stain resistant. Follow the directions on the bottle: usually this means wiping the liquid off after 10 minutes. If you allow it to dry, you have a major scraping job. The plastics used in the sealer break down in sunlight so you need to repeat the application every year or two to keep the stain resistance at its peak. I know the bottle says "lasts 20 years": I think it means 20 years in the bottle.