There are different opinions in catching the 'rakah' of salah in congregation. According to hadith in Bukhari, Muslim and others, the Prophet -Peace be upon him - said; "There is no prayers for the one who does not recite the opening of the book i.e. al-Faatihah". It means that while joining a 'rakah' one could not recite al-Faatihah, this 'rakah' is not accounted for and it is assumed that this 'rakah' is not offered and is to be offered after 'tasleem'.
Some scholars are of the opinion that if one joins the 'rakah' and finds 'ruku' it means he has offered the 'rakah'.
On the other hand there are scholars who opined that if a persons joins the 'sajda' (prostration) of the 'rakah' it means he has offered the 'rakah'.
What ever appeals to your heart, do it. To avoid any doubt and be satisfied, would it not be better to choose the first option.
And Allaah knows the best.