You can use RoundUp but VERY carefully. Here's the trick. Take a big plastic milk jug and cut off the bottom. Then place that directly over the grass area you want to kill and spray into the spout of the bottle and into the grass. The milk jug acts as an enclosed chamber and prevents over-spray from getting to the juniper you want to save.

Check the label on sethoxydim (Vantage) herbicide. I think it is labeled for grass control in juniper

Very clever idea, Money Pit!

Grass-Be-Gone is safe to use around Juniper and readily available.

In my experience, Grass-Be-Gone works well.