Praise be to Allah.
Generally speaking, when a woman attends women’s gatherings, she should be modest in her dress and polite in her speech, especially if it is a study circle, or it is in a school for teaching Qur’an and Islamic knowledge and the like.
But it is not prescribed for women to observe hijab on the grounds that the angels can see them, and that the angels are to be regarded in the same manner as non-mahram men, or for them to do that by way of respect and modesty.
There is no evidence to support this action or to support introducing any rules into the religion of Allah for which Allah has not given permission. If that were correct, then women should wear hijab at home too, because the angels could see her there too.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
Is it true that the angels will not attend women’s halaqahs if the women uncover their hair (i.e., they do not wear their hijab during the study circle)?
He replied: I do not know of any basis for that. They may recite Qur’an and remember Allah, may He be glorified and exalted, even if they are bare-headed, so long as there is no non-mahram man present. That does not prevent the angels from entering." (Majmoo‘ Fataawa Ibn Baaz 24/85).
With regard to the Muslim woman sitting in study circles and Qur’an circles wearing modest clothing, being well-mannered in her speech and dignified in her actions, this is something praiseworthy and it is what she should adhere to, because being well mannered and paying attention to modesty in study circles and Qur’an circles is something that is prescribed and is encouraged.
An-Nawawi (may Allah have mercy on him) said:
It is recommended for the one who recites Qur’an outside of prayer to face towards the qiblah and sit in a humble, calm and dignified manner, with his head lowered, and he should pay attention to proper etiquette in his manner of sitting, as he would when sitting before his teacher. This is the most perfect option, but if he recites standing or lying down on his bed, or in any other posture, that is permissible." (Mukhtasar at-Tibyaan p. 17).
And Allah knows best.