Elsewhere1. Sign the device out
Open your account on elsewhere.is, find the device in the list, and choose Sign out. If you can't tell which one it is, choose Sign out everywhere, then sign back in on your own devices.
Signing it out ends its access. It can sign in again only with a new code from your email, so secure your email next.
To be sure it's cut off right away, turn off sharing on your computers from the menu bar until you've finished these steps.
2. Secure your email
Elsewhere has no password. It signs you in with a code sent to your email, or with a passkey you added. If someone signed in without you, the likeliest way in was your email.
Change your email password, and turn on two-step verification for your email account.
3. Check your computers
On each Mac, the Elsewhere menu shows who is connected right now. Disconnect anything you don't recognize.
In Settings > Sharing, choose Ask on this Mac first, so every new session needs a click at the Mac. Once that is set, turn sharing back on.
Common questions
Why did I get this email when it was me?
Every new device that signs in to an app gets one, including your own. The first device on a new account doesn't. Signing in on the website doesn't send one.
Does Elsewhere ever ask for my sign-in code?
Never. Anyone who has your code can sign in as you, so never share it.
Could they have seen my screen?
Only while connected to a computer with sharing on. Each Mac lists its sessions in the menu bar, and notifies you when one starts unless you turned that off.