Is Meiyn Sheva recited in a private Minyan that is not taking place in a Shul?
The blessing of Meiyn Sheva is only recited in an established Shul, which is defined as an area in which a Minyan Davens on a constant basis. [This applies even if the Shul does not contain a Sefer Torah.] However, in an area that a Minyan Davens only on mere occasion, such as those that make an occasional Minyan in their home [such as in times of rain], then the blessing of Meiyn Sheva is not to be recited. Nevertheless, those areas that are accustomed to recite Meiyn Sheva even by a Minyan of mere occasion, are not to be protested. [In Yerushalayim, the custom is to recite Meiyn Sheva even by an occasional Minyan taking place in a private home. According to Kabala, every Minyan is to recite Meiyn Sheva irrelevant to where the Minyan is taking place, even if it is a mere one time location. Practically one may not follow this custom unless he practices all of his rulings according to Kaballah.]
A Minyan in the house of an Avel or Chasan: The Minyan that is accustomed to take place in the house of an Avel or Chasan is not considered a set Minyan and hence Meiyn Sheva is not to be recited.
A Set Minyan that will last a few weeks: If a set Minyan will take place in a certain area for a limited amount of time, such as those who make pilgrimage to certain cities and set up a Minyan for a few weeks, then there is an opinion which rules that Meiyn Sheva is to be recited there.
Is a set Shabbos only Minyan to recite Meiyn Sheva?
Yes. Some Poskim write that this applies even if the set Minyan only takes place on Friday night.
Is Meiyn Sheva to be recited by a family Minyan taking place in a Simcha hall, or in the house of one of the Mechutanim?
No. Some however write that if the hall contains a set shul which serves a different Minyan each Shabbos, and it contains a Sefer Torah, then it may be recited.
If a set Minyan has temporarily relocated, is a Meiyn Sheva to be recited in the new temporary location?
Yes. This applies even if they are Davening in the street.
 268/15; Taz 268/8 based on Michaber 268/10; Mahriy Abuhav; Ruvash 40
 Noam Megadim Minhagim 9; Birchas Habayis 41/7; Dvera Moshe 1/32, brought in Pischeiy Teshuvah; Igros Moseh 4/69; Minchas Yitzchak 10/21;
 The reason: As since the area does not contain a constant Minyan the reason behind the recital of Mieyn Sheva, which is to allow the latecomers to finish their prayer [and not return home alone], is not applicable in this scenario. Now, although in today’s times this reason is no longer applicable even regarding the town’s Shuls, and nevertheless it must be recited, this is because the institution of the Sages does not become nullified even when the reason behind the institution becomes no longer relevant. However by an area without a set Minyan the Sages never instituted to recite Mieyn Sheva since the reason is not applicable, and hence one is not to do so. [Admur ibid; Taz ibid]
 Admur ibid; M”A 268/14; Maharlbach 122; Ridbaz
The reason: (As this matter is not mentioned in the Gemara or the words of the Geonim). [Admur ibid]
Other opinions: Some Poskim rule one is to protest against them being it is a Safek Bracha Levatala. [P”M brought in M”B 268/25]
 Ben Ish Chaiy 2 Vayeira 10; Eretz Yisrael of Rav Tukichinsky 5/3; Har Tzevi 1/152; Minchas Yitzchak 10/21
 Menora Tehorah 268/11; Rav Poalim 3/23; Ben Ish Chaiy 2 Vayeira 10; Kaf Hachaim 268/50; See Likkutei Maharich
 Kaneh Bosem 2/48; Or Letziyon 2/19-5
 Admur ibid; Michaber 268/10; Mahriy Abuhav; Ruvash 40
 Admur ibid
 Taz ibid; M”B 268/24 in name of Elya Raba limits this to only if the Minyan has a Sefer Torah in the temporary Shul.
 The reason: As also by this Minyan there are people that come late. [ibid]
 Birchas Habayis 41/7; Igros Moseh 4/69; Minchas Yitzchak 10/21; Shraga Hameir 6/47; Mishneh Sachir 1/91
 See Poskim ibid
 Piskeiy Teshuvos 268/13
 Piskeiy Teshuvos ibid footnote 98
 Tehila Ledavid 268/13; Minchas Yitzchak 10/21