We are required to have a security person at the door when the building we daven at is otherwise closed (on Shabbat). For financial reasons, we use rotating volunteers, but sometimes, like for Mincha, we have only ten men. The security person can just barely see into the shul room from the back door of the room where we daven. The last person in the room can barely see them. Can that security person be the tenth, bedieved or lekhatchilah?
Shulchan Arukh (OC 55:14) rules that a tziruf for minyan can be achieved even if a person is outside the room / outdoors if he can be seen by those inside. This is based on the established ruling by zimmun that if some of the group can see some of the other group, it is considered one group for zimmun (Shulchan Arukh OC 195:1). While some argue against the comparison and are stricter by minyan(mentioned by Mishnah Berurah, see Arukh HaShulchan OC 55), the accepted psak is to rely on Shulchan Arukh when necessary. You have to situate a man in the room inside so that he can see and be seen by the man outside. That is key. If you deem it important to sometimes have a man do the job, then yes, you can definitely follow the ruling of Shulchan Arukh, but with the criteria that both men must be able to see and be seen.