I can think of two possible reasons why you're asking this question:
1) If you are trying to have a consistent .gitignore across users, you just have to add and commit the .gitignore file to your repo.
2) If there are files showing up in Bitbucket that you want to ignore, .gitignore is not going to solve the problem. If files are in Bitbucket, then they're already being tracked by Git (you've committed them). .gitignore is for hiding uncommitted files. You'll have to remove those files from the repository, and commit the removal.
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