Matt 18 actually gives the answer. Keep reading:
Mat 18:27 Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt.
Mat 18:30 And he would not: but went and cast him into prison, till he should pay the debt.
Binding and loosing a person is forgiving a person or not forgiving a person. Literally, it means requiring something or not requiring it.
Jesus gave us that much authority that when two or three of us make a decision on a person's state, being forgiven or not, due to the reaction of requiring one to repent, that it is as though Jesus made the decision. That is what it means for two or three to gather in his name and see Him in their midst. It does not mean if only two show up for church Jesus is there.