Is it lifnei iver to recruit someone to attend a minyan where there won’t be 10 mispallelim, without informing him that according to many poskim (MB 90:28) it is not a valid minyan (according to others it is valid)? Should one inform him that they are relying on a controversial opinion that it is tefilla btzibbur even though according to Rav Moshe it is not?
It is not lifnei iver to ask the person to daven there. It is definitely better to daven with ten people that are starting to daven together, because there are those who say that we need specifically ten people starting to daven together. However many poskim, including the Mishna berura hold that it if there are six people davening together the person is yotza tefillah b’tzibbur. Additionally, the person helping the minyan is also doing a chesed and allowing the other people to daven. Therefore we can not say that someone who is helping out this minyan is doing something wrong, and by asking him to join that we are being misleading him.
As a side point, the Mishna Berura that you are quoting, is not discussing what is called a minyan regarding tefillah b’tzibbur, but only saying that a person cannot daven by himself, thinking, “Oh, there are ten people in the room now, so it is tefillah b’tzibbur”. Also see Mishna Berura 69-8, that if six people are davening together it is considered tefillah b’tzibbur.