Praise be to Allah.
We put this question to Shaykh Muhammad ibn ‘Uthaymeen, may Allah preserve him, and he answered as follows:
, Lord of the Worlds, and peace and blessings be upon our Prophet Muhammad and all his family and companions. There is nothing wrong with this announcement, because it is a call to good and it does not involve buying or selling. What is haraam is making announcements about buying, selling, leasing and renting, which are things for which mosques are not built, but there is nothing wrong with announcements that call for good, feeding others and acts of charity.
With regard to the question of whether this is a kind of joint worship that is not prescribed in Islam, the fact is that they are not calling for communal fasting, they are just announcing that iftaar is available, and there is nothing wrong with that.
And Allah knows best.