Only users level 15 and above may create invite codes. The limit is up to 5 codes per user and all codes are single-use.
To create an invite only you should go to your settings page (gear icon) and select "invite codes" in the sidebar. Alternatively, if you're too lazy you may click this link
Also: Talking about Test 4 is strictly verboten. Please remember this.
Fear not, I shant allow anyone past me!!
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The fact that I have no friends and have no one to invite should have no impact on the spectacular virility of my above comment!!!