Since you mentioned that your actual friends are not happy with her intrusions why not use their quite reasonable objections to her pushy behavior to explain that you'd prefer she not join your group socializing because your friends have asked you to discourage her from doing that? This is somewhat kinder than telling her you don't want to spend time with her outside work because you can present it as saving her from the embarrassment of being cold-shouldered by people who are definitely not her friends and who have no wish to be. If worse comes to worse perhaps one of your braver real friends can be persuaded to bluntly say that this is a private gathering to spend time with you and not your casual acquaintances next time she just sits down uninvited or invites herself along to anything.