I say, and with God's success: The one who is considered present is the one who catches all the prayer with the imam, and the one who is late is the one who misses part of the prayer after catching the beginning of the prayer with the imam; he must perform it directly and then complete with the imam. The one who is behind is the one who misses something from the beginning of the prayer, so he should perform it after the imam's salutation. And God knows best.