Given that the symbolism of this holiday resonates so nicely with the other holidays of this season (the light symbolism of the solstice celebrations, the Christmas star, the candles of Hanukkah, etc.), it seems like it would be possible for people to join in on this celebration easily, sharing this holiday together with others from many different faiths or for those who are secular without a professed faith at all. Given the current climate, however, I'm sure that there are many people who would perceive this holiday not as a complement to other holidays, but as a threat to them, alas.
One thing in particular I would like to give the organizers of HumanLight credit for is that they have a page at their website where they address the many different concerns and complaints that people, both religious and secular, have raised about the holiday. A static webpage is not exactly a dialogue in and of itself, but it is a strong gesture towards a dialogue, and I found it full of interesting questions and ideas.
Speaking for myself, I like very much the idea of celebrating the winter solstice, even though I am not a Wiccan, because the sheer natural profundity of that moment in astronomical time is really striking to me. I find the Wiccan wheel of the year, built on the solstices and equinoxes, to be profoundly appealing. So, in my own personal calendar, you'll find me organizing observations of those "wheel of the year" holidays. At the same time, if somebody invited me to a HumanLight celebration, I would definitely attend - I think it's a lovely idea, and I respect these people's efforts very much!