Here's the deal people:

IF YOU WERE "RANDOMLY" INVITED,
Then that means that you played with [Constant] and he liked your playing and/or you (99% are people who use mics). He invited you to join Myriad, but you are not obligated to join, [Constant] won't be offended if you decline. If you are invited by [Constant] though, it is recommended that you accept because....

IF YOU WANT TO JOIN MYRIAD,
You can only do so by playing with [Constant]! If he thinks you're fun to play with, he'll invite you. Note that there will only be a SET AMOUNT OF PEOPLE who will be in Myriad at one time. The limit will probably be around 20 or 30. Basically, if Myriad is full, [Constant] will have to kick someone out before letting someone new in.
Them's the rules.

Dammit Valve