When you sign in to CircleCI, you can do this either with your email address or with one selected VCS provider (example: Github).
If you signed up with your email, you will be offered the option to connect your Github or Bitbucket user to your CircleCI account. If you signed up with a VCS provider, you will be able to add additional account integrations (example: BitBucket) on your account's page here: https://app.circleci.com/settings/user.
If you have already linked or signed up with a VCS user to CircleCI previously and are trying to connect this same user to another account, you will see the following error:
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.
If you receive this error, please contact Support and we will help you unlink the user from your previous account, so they may be rejoined as you wish.