Performing the sunnats of fajr when the Jamaat has already commenced

If he is sure that he will be able to perform his fajr Sunnats and still get the Jamaat, then he should perform his Sunnats in the sehn of the Musjid (i.e. in such a place that he will not be an obstruction for those who wish to join the saff) and thereafter join the Jamaat.

However, if he fears that through performing the Sunnats, he will miss the jamaat, then in this case he should not perform the Sunnats, instead he should join the Jamaat.

And Allah Ta'ala (الله تعالى) knows best.