Join The Club Bring Me The Horizon

I feel this song is about people needing to realize they are not the only ones that are having struggles. The title and repeated lyric "Join the club" is enough to make one believe this song is about realization that others have similarities.
More directly this song seems to speak directly to those who might not be in the "IN" group, those who feel they are so different and dealing with so many struggles. The lyrics are to both clarify that 1.) they are not the only ones dealing with those issues or struggles and 2.) should stop making overly dramatic statements about everything - "just let me sink, let me sink into the fucking ground"..."fuck it". To me that is almost a call and response from someone looking for attention and someone who is tired of trying to help that person (which would explain the drop and softly spoken "fuck it" as if they are giving up helping them).
Additionally I believe this song is intended to provide peace of mind to those dealing with issues mentioned in the song (or any issue for that matter). 
To me this is an example of a situation where people who do not "fit in" realize they do not need to, they fit in with each other and that is enough.

Lastly, my favorite line of this whole song is "We're all fifty shades of fucked up". When I first heard this song it was just background music while I was typing a paper, so I wasn't exactly paying attention to the song, but as soon as Oli screamed this line I popped my head up and raved about how true that was as well as it being a fairly clever line. 
I see this lyric as the point to really ease the mind of who it is directed to, it is like saying "you have your problems, I have mine, he has his, she has hers, we ALL have problems, it does not matter who or what you are we all have problems and struggles and that is perfectly fine!" It is a way of showing someone they are not alone. Also, I am almost certain this was written as spoof of the Fifty Shade of Gray book series, which could provide an secondary meaning. That secondary meaning being directly related to the novel series itself, the fact that today's society is so enthused by sex that a book that glorifies sex, more specifically bondage, becomes a chart topper, bestseller, etc. 

This is all my personal opinion, hopefully it can help you gain an understanding or give you an idea of what the lyrics mean.
It is not a complicated song to understand but I feel some people might over look subtle possibilities about it.