Are you an OnDemand/JIRA customer? If so, you may have the DVCS connector configured to automatically add/invite users to your repsoitories. Check out the option for Configuring default groups.
If this isn't it, please let us know your username on Bitbucket and we can take a look and reply here, or via support at support.atlassian.com.
Marcus - Bitbucket Support
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