If the person from the congregation makes the takbeerut al ihraam standing, then he goes into the rukoo’ and he catches the imaam in rukoo’, then he has accomplished that rak’ah – according to the hadeeth of Abu Bakrah (radi Allaahu anhu) that he reached the Prophet (sal Allaahu alaiyhi wa sallam) whilst he was in rukoo’ – so he (Abu Bakrah) went into rukoo’ before he reached the row.
So the Prophet sal Allaahu alaiyhi wa sallam said to him:
May Allaah increase you in eagerness and do not repeat it (ie the rak’ah). - Bukhaari reports it and Abu Daawud made an addition to it:
And he bowed whilst not in the row then he moved to the row. - (Abu Daawud 1/157, Ahmad 5/39, Bukhaari 1/188 and others)
And due to what is reported by Abu Daawud from him sal Allaahu alaiyhi wa sallam:
Whoever catches the rukoo’ has indeed caught the rak’ah. - (Abu Daawud 1/206)
And with Allaah is the tawfeeq. And may peace and blessings be upon our Prophet Muhammad and his family and his Companions.