There is spam, but its all contained into one area. So with that in mind that’s not really an issue. Also by looking at how the posts are written it allows us to see just who we should look at as a potential member or who we should avoid.
It pretty much is an invite only as far as someone having a high chance to get in. But with this current system those who want in can state they want in, and have the chance to prove themselves (from us watching them).
So I say the current method is fine. It allows for easy adjustments (we can simply stop inviting people as quickly when the clan starts to get to full ect.)and has proved effective in spotting good members amidst spam. If someone spams in the thread and appears generally unintelligent/immature... then they pretty much not only killed their chances to get into Guru but most any other clan that isn't desperate.
Again keep in mind that list only serves to spot "potential members", just to see who all we have interested. The highest chance for someone to get in Guru lies with recommendations and... us simply being greatly impressed with their individual skill and asking them.
Also the invite post can demonstrate dedication; there have been members that were invited primarily (their skill ect. was also a factor) because they patiently waited for months to try to get in.