There are a few important points to make about the verb. First, it is in the passive voice, which means that the stones are not assembling themselves into the wall but are the objects of God's activity. Second, the verb involves ongoing activity, indicating that the Lord God is continually constructing his building with the addition (and perhaps tweaking) of stones. Third, the implication of the action described here is that living stones (Christians) do not lie scattered hither and yon across the landscape so as to have nothing to do with each other; instead, they form a house (i.e., unity) where each stone contributes to the greater whole (think here of the communion of saints).
5 you yourselves like living stones are being built up as a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ.