What is the ruling on a niqabi woman working as a makeup artist, given her desire to adorn women (niqabis and non-niqabis), and is it permissible for her to display pictures of the adorned women in a closed group exclusive to women?

The basic principle regarding beautification and adornment is permissibility, provided it is not accompanied by anything unlawful (such as impure or harmful substances, or things that prevent ritual purity, or adornment in the manner of people of indecency), and provided that the adornment is not displayed before non-mahrams. Closed online women's groups are not entirely safe for posting pictures of women, especially if the focus is on charms and adornment. However, displaying a specific part of the face (such as an eye or a lip) does not reach the degree of prohibition unless it incites fitna. Piety in these matters is safer for a woman's religion.