Do I need to do anything to opt in?

No, the accounts of customers who do not expressly opt out will automatically transfer to Betterment on the Transfer Date.  There is no need to create a Betterment account in advance of the transfer, though you are welcome to do so if you choose.  Account assets will not move from Marcus Invest to Betterment until the Transfer Date.  Betterment will send additional instructions for claiming your Betterment account closer to the Transfer Date.