This help article on marriott.com seems to make it like you can already start earning points on citizenM stays:
Q: Can I earn Marriott Bonvoy® points on citizenM hotel stays?
You can earn Marriott Bonvoy® points on citizenM hotel stays by adding your Marriott Bonvoy® account number to your reservation. You can do this at check-in.
Q: How many Marriott Bonvoy Points and Elite Night Credits (ENCs) will I be able to earn at citizenM?
Members earn 10 Marriott Bonvoy® Points per $1 USD and 1 Elite Night Credit per Qualifying Night.
Has anyone tried it yet or are they just preparing for the integration by creating articles beforehand? I read on one of the bloggers that they would fully join Bonvoy on Oct 28, but I haven't gotten any confirmation in a citizenM or Marriott release.
The article also mentions that Platinum and above will get 4pm checkout (which is nice, because they currently charge about £20 for 2pm one - at least the London Victoria one) and the welcome gift is 500 pts or $10 F/B nightly credit, so like like AC/Moxy no free breakfast.