Basically your issue will be clumping. Guar gum powder and most other fine powders are so fine (micro fine ) they insist on clumping together. The way round this is either, as previously mentioned, blend it which will likely cause a massive amount of air bubbles OR the preferred method (for chef's ) is by using equal amounts of maltodextrin, which is basically processed starch somewhere between it's original state and glucose. As a result it has a purposely 'nothing' taste, neither sweet or savoury. This combined with any micro fine powder prevents clumping (just check the pack of your preferred gravy thickener). As a trained pastry chef this is a product I've used for years.

Source: the kind lady at MSK held a introduction course to their products.

OR Just Mix the powder with a bit of icing sugar and hope for the best. There's no real reason this shouldn't work but I've never tried it. It also will sweeten your sauce/juice a bit.

Edit

Sorry I missed the part about 100% fruit juice. I'm sure there is work arounds I.e use a fruit derived sugar?

Edit2

Can you say 100% fruit juice and still be adding guar gum?

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.

I agree with Sourd'oh, your best option is to use a blender. A mixer will do too. A high RPM number is good, but not essential.

What works well is to blow the powder gently onto the surface of the juice. I do this with a handheld mixer: I hold a teaspoon with the colloid builder (guar gum or other clumpy substance) above the surface, and either tip the spoon very slightly or let the mixer exhaust catch just a few grains of it. It lands on the surface in a single layer, not in a clump. The mixer beaters take it and swirl it, and it gets well dispersed. Repeat until there is no powder left.

I'm afraid this might be quite time consuming if you are talking about industrial quantities, but your other option is dilution as described by Doug, and this breaks your "100% juice" requirement. Maybe you can legally get away by diluting by using some powder derived from the fruit, but anything which consumers will not consider "cheating" will change the texture for the worse.