Patchou has many many friends who he sends test versions to if he needs an update testing. I guess if you manage to become really good friends with him then you'll become a tester.
When he was developing the first release of MP2 there were a lot more testers who were signed up to do extensive testing. Until (if and when) there is a Messenger Plus! 3 I doubt very much that testing on this scale will be needed.
To be honest, I don't see the need to be a tester any more
... there's nothing particularly interesting to test and what I have on this version of plus is more than I could ask for
apart from the display picture bug and keeping the personalised status suffix and waiting isn't hard