The test group is around 1000 Android users - the update is live on the Beta channel but as those users are already selected, if you’re not on the list then don’t feel the need to rush into the Beta channel and enable it, as it won’t make a difference and it’ll hit the stable channel in a few days anyway.
Those already on the Beta channel and who have been selected will receive a migration notification in the app.
Regarding feature parity - yes and no. P2P (Monzo-to-Monzo payments) is there. Monzo.me isn’t yet. The reason for this is simply down to time and prioritising. P2P is used by almost all Monzo users, Monzo.me isn’t, and with current account, you can get instant bank transfers anyway if you need to a payment from a non-Monzo user. If monzo.me is something you use heavily, then feel free to not migrate at this stage.
Regarding criteria - there are some, but I’m afraid I’m not totally up to speed on what they all are. It’ll likely be down to ensuring a decent mix of users - for example it wouldn’t be very helpful to only test on the same Android version and similar devices 