Al salam alykom..
if i pray (ma'mom) and the immam finished the salaht.. but I didn't so I stood to complete
my salaht .. then a person came to pray with me as a ma'mom ? Is my immamah correct?
wa jazak allah kyran.
If a person joins you wanting to pray with you, and you make the intention of leading him in prayer, this is OK.
The author of the book Nihaayat al-Zayn fi Irshaad al-Mubtadieen (p. 127) said: If the imaam says salaam then one latecomer decides to follow another latecomer in prayer, this is OK.
And Allah knows best.