How does Groot spread light?

Within the Comics (The Bendis versions at least, which is the most current run of GOTG), groot actually has bulbous, glowing areas as par of his anatomy: so there is a precedent for bioluminosity...

By the look of the glowing spores he produces, it is probably to suggest that groot (being a plant) disperses his seeds as by driftseed. There are hundreds of plants that do this, and the relative difference in gravity could account for their 'floating' effect, if they are not self powered somehow (this is an alien species, after-all).

Either way, I think its safe to assume the 'glowing things' are supposed to be seeds, and Groot is dispersing them to produce light...

Since it's not in the comics there's no explanation there where one would usually turn.

It's not explained in the movie either.

I also cannot find any official announcement from the movie creators explaining what they are.

All these things mean it's down to interpretation, i.e. your question cannot be answered at this time.

My interpretation is that a few of them appear to have wings, so probably glowbugs. But they could also very well be large balls of bioluminescent cells (like what glowbugs have), since it would seem more lore-friendly to me that Groot can only produce flora, and there are some moss' with bioluminescent cells.

How they float is another thing entirely.