1) Be nice.
2) When posting content, it should respect the privacy and rights of others.
3) Piracy, pornography, or any other material that may be inappropriate to a general audience should not be posted to the site.
4) While the site gives you a lot of freedom for writing posts, do not abuse that. Keep your posts related to gaming and try to keep your posts readable. Also, if your post has too many large screenshots, try embedding only one of them and linking the others, or reducing their size, or use HTML to come up with a creative way to alternate between them.
5) Do not spam.
6) Do not double post.
7) Do not beg users for gifts or ask for any kind of donations.
8) External links are fine as long as they do not contain referrals. If you want to promote your own website or any kind of project, you can include it in one of your regular posts only once (see guideline number 5). You can have the link displayed on your profile, but don't try to circumvent this particular guideline by repeatedly telling people to look at your profile to check out your website/project.
9) Do not enter group-exclusive giveaways if you have not been active lately. If you do, you will be kicked out of the group.
10) Once you apply for Steam group membership do not keep asking for updates on your application.