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You're not allowed to post external gibs where users have to do something like join a group to get something.
Edit: From the guidelines:

When posting links or content, that content should not force users, encourage users through reward, or primarily exist as a traffic source for users to perform an action for promotional, commercial, or monetary benefit. Such actions include but are not limited to clicking a referral link, liking a Facebook page, following a Twitter account, joining a Steam group, completing a survey, or making a donation. (Please note, in giveaway descriptions we do allow users and developers to link to their related Steam Greenlight, group, and store pages, as well as their social media channels. Users are also able to reference their group giveaways when posting in the Group Recruitment category of our forum.)