What is the difference between communal Dhikr and reminding others to engage in Dhikr, and is sharing Adhkar such as "Astaghfirullah" and "Salla 'ala Muhammad" via social media considered a reprehensible communal Dhikr?

Gathering for remembrance (dhikr) at a specific time, limiting it to a certain number, and repeating it after one person is an innovation (bid'ah) that was not practiced during the era of the Righteous Predecessors. As for enjoining dhikr, encouraging it, and reminding people of it, these are not prohibited, nor do they fall under the innovated collective dhikr.