I am at the point in the Knighs of the Nine questline when several people have asked me if I was to accept them joining the faction. I have already let Lathon join the guild and two or three others have asked me if I was to let them. Is there any pattern when is it the right moment to let them join the faction or some kind of point when it is better to do so than another time? Or is there no difference whether or not I let them join whenever they ask?

The main question is whether or not you want them with you when you do the big fight at the end. They can be killed during that battle if you allow them to join before it. In fact I've never managed to have Sir Avita survive it although if you're quick enough the others can. Sir Rodric's comanion will accompany you to Underpall cave if you allow him to join before you do that. I prefer to go alone.

Any named knights who die will be replaced by a generic knight who will be less well equipped. So if you want to be sure all the named knights survive don't recruit untill you've finished the questline.

Mara

Dang, I've allowed them in and now eight of them are at theGarlas ruin waiting for my sign. I ordered them to stay put but they all run in when I enter the door. And there's no stopping them! They just head down the stairs and I think they will eventually meet the "curiosity killed the cat" end. Luckily I made some restore health on touch spells and have the shield/restore health spells from the crusader equipment. I'll try to keep them alive.

just run through the ruins as fast as you can, when you break the crystal your guys are safe.

Thankfully the quest ended successfully. That was one of the coolest quests I've done recently, well thought story line and idea, a great official plugin.