Importance Of Adhan
If the objective is to call other people for congregational Salah to the mosque or some other place designated for congregational Salah, then Adhan would be required in additon to aqamat. But if a group of people just want to hold Jamat at any place, then only aqamat would be necessary. As for Obligation of Praying in a Mosque , please refer to the videos of Ghamidi Sahab shared in the following thread: Discussion 83388