Though the NIV translation mentions that all the believers met together in the Colonnade, the original Greek text does not say this. In the Greek we are told that they
met together in the Colonnade. This could certainly be a reference to all the believers, but it is also possible that the apostles specifically are in view. That is to say, the apostles met together regularly in Solomon’s Colonnade, which is also where they performed miraculous signs.1 It is thus not that clear from this verse if the apostles or the whole church was meeting at Solomon's Colonnade.2
12 Now many signs and wonders were regularly done among the people by the hands of the apostles. And they were all together in Solomon’s Portico.