As Salamu Alaykum If I enter the masjid late when the congregational prayer is already over but there are people making up their missing raka'at, should I tap one of the people on their shoulder and join them to make a 2 person congregation? or should I pray on my own?

Ustadh AbdulAziz al-Haqqan

You can, and you can wait for someone late to come and pray with him.