I find blind invites highly annoying as well even when it comes from someone whom i know.
But what i find the most annoying is tells like "hi m8 can i join u on ur team plz"
First off. Take the few seconds it takes type "hi mATE can i join YOu on YOur team plEASE" its not that hard!!.
Second off. the please in the end makes it sound like begging in my ears and is something i find just as annoying as people begging for cash.
Third off. I know not everyone is every good at English (I know I'm one of them!!) But at lest try to be polite and do your best when typing, It gos ALONG way in my eyes.
I rather join and help an low lvl or have an low lvl join me, IF the person asks politely BEFORE HAND.