Overview
You can sign in to CircleCI with an email address or with a VCS provider (GitHub or Bitbucket).
“Merging” accounts usually means putting the VCS identities you need on one CircleCI user, for example:
You signed up with email and want to connect GitHub or Bitbucket
You signed up with GitHub and want to add Bitbucket (or the other way around)
You have two CircleCI logins and want one of them to own a VCS identity that is currently on the other
You can connect one account per VCS provider to a single CircleCI user. You cannot attach two different GitHub users (or two Bitbucket users) to the same CircleCI account. See Can I merge two separate GitHub accounts on CircleCI?
Solution
Connect a VCS to the CircleCI account you want to keep
Sign in to the CircleCI account you intend to keep.
Open User Settings.
Under Account Integrations, connect GitHub and/or Bitbucket.
If you signed up with email, use Connect on the GitHub or Bitbucket integration.
If you signed up with a VCS, you can add the other supported provider from the same page.
If you see “identity already associated with a different account”
Connecting a VCS user that is already linked to another CircleCI account fails with:
We encountered a problem adding your identity to CircleCI… The identity provided by your VCS service is already associated with a different account. Please try logging in using that account or contact us for further help.
To free that identity and attach it to the account you want to keep:
Sign in to the other CircleCI account (the one that currently owns the VCS connection).
Disconnect the VCS identity by following How to Manage Your VCS Connections with CircleCI and delete your user identity.
Sign back in to the account you want to keep and connect the VCS again from User Settings.
After you disconnect, you may need to recreate personal API tokens on User Settings → Personal API Tokens.