how can I get guar gum to smoothly blend in juice?
The lumps of gum in products are known as "fish-eyes". Unless you get pre-hydrated gum, the best way to avoid them is to mix it in a blender (and a pretty powerful one at that). Sometimes pre-hydrated gum will mix in more smoothly and with fewer lumps, but you'd still need to blend it. Another option that may not be practical in your case is to disperse the powdered gum in another powder, like sugar, and then mix the blend in.

Here are a couple of sets of instructions on mixing gums.