What is the ruling on joining mixed-gender Islamic groups on Facebook, and is it permissible to comment on or like the posts of brothers in them?

Joining Islamic groups is permissible, and may even be recommended if it involves benefit or cooperation in righteousness and piety. Caution must be exercised in interactions between genders, ensuring there is no private communication. As for commenting on posts and liking them, it is permissible as long as the post does not violate Islamic law.