Question

During reserve duty I encounter quite a bit the familiar phenomenon of being asked to complete a minyan—people ask someone to be the tenth for them (for Mincha/Ma'ariv), and even though he isn't really interested, he does it as a favor, or because he feels uncomfortable being the one who prevents them from praying with a minyan. So I wanted to ask the Rabbi what, in his opinion, is preferable (halakhically and morally): to pray with a minyan when part of it consists of people whom you pressured into joining so you could complete it, or simply to pray alone? And a second question: is there any halakhic obligation on an individual to be the tenth person for a minyan?
Thank you