The Harbingers of Light are members of the Tuatha. I highly recommend reading about the Tuatha and how it members are connected to Gaiscioch. That said, here is how I view Gaiscioch and the Tuatha.
The Gaiscioch Family is just that, Gaiscioch. You may want to call them a Guild, but frankly it so much more. They are a huge family of Gamers, Role players, and Hobbyist all of whom meld together in the common mindset of "We not Me".
Now let's look at the Tuatha. Members of the Tuatha are members of the Gaiscioch Family, but we don't always fight under Gaiscioch's banner. We are the Harbingers of Light. We adhere to our our own creed that is separate from Gaiscioch's Credos and the Tuatha's guidelines of "Blades out, Not In". So that means being a member of the Harbingers of Light requires you to conform to our guild philosophy AND follow the Tuatha's guidelines of "Blades out, Not in". You are not however required to accept Gaiscioch's Credos or use Gaiscioch's Website (but honestly why would you not, LOOK AT THIS PLACE!).
Now you may be thinking, but Pookha!?!? you're a "Caomhnoir na Ulchablah blah blah (Speak Murican!!!!)" in Gaiscioch so are you a member or not? The simple answer is yes! I'm a member of the Gaiscioch Family just like you. I accepted the same Gaiscioch Credos you will accept as a Gaiscioch member. The only difference is I bleed Green and Gold :D. As a member it is your job to define your membership to the Gaiscioch family as you see fit "Play how you want, When you want" just remember your credos. One of the main advantages of the Tuatha is that my guild members have full access to the Gaiscioch family. They are able to become full fledged members of the family and still fight under the HoL banner.
So what about full time Gaiscioch members joining the Harbingers of Light? I'm all for it. In fact we have quite a few members of Gaiscioch within our ranks.
So why join the Harbingers of Light in the first place? Well that simple too. It's about what you bleed? we are a small close knit family for many that's what they are after. Every member knows every member. For some people that's just what fits for them and a family the size of Gaiscioch is just to much for them. It's all about what you are looking for in a guild.
So if you think you are worthy to bleed Green and Gold drop us a application.
I hope this helped answer some questions and I look forward to seeing you on the battlefield :D