You cannot roll over a spouse's IRA into your Betterment account, as the IRS does not allow for joint ownership of IRAs. Your spouse will need to create their own Betterment account to roll their IRA into.
If you and your spouse want to sign up for individual accounts, we can "household" the two accounts which will combine your balances for the purposes of meeting minimum balance requirements at our various pricing plans. Please email us with both of your email addresses if you would like to “household” your accounts, and we will make the necessary changes on our end.
Additionally, we do allow you to add account beneficiaries. This would give the indicated beneficiary immediate ownership and transfer rights should anything happen to you. You can edit this at any time by clicking "Settings" on the top right hand side of the page, then "Accounts" from the sub-header. Once on this page, you will enter the beneficiaries information for the individual accounts.