I am of the opinion the church had it's nucleus of formation in place before Pentecost, and was empowered at Pentecost, but it was far more than the twelve, as we see in Acts 1:15 that prayer meeting contained 120 people.
I agree there was empowerment. No question in my mind. However, that meeting was as a result of the people meeting in anticipation of the coming of the Spirit
rather than it happening at their weekly church service.
I am certainly not stating they had weekly meetings as we see them today, and the first day of the week did not become the established day of worship until after the resurrection, see in actuality they met corporately daily (see Acts 2:46) at least for a time, and it is true that they were waiting for the promise of the Father, specifically the empowerment of the Holy Ghost, but my statement about the formation of the church comes from:
Matthew 18:15-20. The word used was not in a "universal" or "institutional" sense, it is instruction for a local assembly, it is the greek word, "ekklesia", the called out assembly.
and
Matthew 16:18, where Christ said He Himself would build His church, now I get the trinity, but He is not here, the Comforter is, so for Himto be able to say this, He had to build it.
Which he did according to
Ephesians 2:20, he laid the foundation of the church with Himself as the head and the apostles as the rest of the foundation, and the the body consisting of those who gathered in that upper room
and
Acts 2:47, those that were saved were added to the church. Some would say "church universal", and while the English word church may allow for this, the underlying greek does not.
On the flip side, this has been an unsettled issue between believers for a long time. Prolly not going to settle it for eternity here.
And while staying with them he (Jesus) ordered them not to depart from Jerusalem, but to wait for the promise of the Father, which, he said, “you heard from me; for John baptized with water, but you will be baptized with the Holy Spirit not many days from now.â€
You can define "small group" any way you want, but we can't have a "small group" meting of 120 in my house. 120 is a reasonable sized church, particularly if you are taking adults.
In Acts, they met "from house to house". The coming of the Spirit on them happened, not in anyone's house but in the "hotel meeting room" as we would look at it today.
Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day's journey away. 13 And when they had entered, they went up to the upper room, where they were staying...
The disciples were "staying" there. Where was Peter's wife? Was she living there too? I'm thinking it was probably the same location where they celebrated Passover with Jesus a month and a half before but that is strictly conjecture on my part.
It could well have been the same upper room, I however disagree with the translation you used. Mine says :where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James. That's a pretty specific group. Peter's wife? Not sure. Conjecture, she had passed away? It is a little like asking where was Joseph at the crucifixion.
It is my belief after study that their idea of house and our idea of house are different. A Roman domus was far larger than a typical North American or even European house.
He said to them, “Behold, when you have entered the city, a man carrying a jar of water will meet you. Follow him into the house that he enters and tell the master of the house, ‘The Teacher says to you, Where is the guest room, where I may eat the Passover with my disciples?’ And he will show you a large upper room furnished; prepare it there.†And they went and found it just as he had told them, and they prepared the Passover.
Either way, the disciples were staying together at that location and they went to the upper room to pray. Do your church members stay together in the same compound and go to the designated "meeting" place each week?
Again, there is NO record these 120 met with each other
every week for a church service.