Is it permissible to join a Facebook group that publishes pictures of women and romantic pictures for the purpose of enjoining good and forbidding evil?

It is not permissible to be present in a place that involves a reprehensible act, except for one whose presence serves a legitimate purpose, such as enjoining good and forbidding evil. In such a case, his presence is permissible to invite them to Allah. So, whoever has the ability to influence others and not be influenced himself, there is no harm for him to enter such sites with the intention of inviting people to Allah. But whoever fears that he might weaken and be influenced by falsehood and its people, it is obligatory for him to avoid entering and participating.