Is it permissible to say "Ameen" after someone's supplication: "O Allah, make us among Your believing servants"?

There is no harm in supplicating with "O Allah, make us among Your believing servants or the righteous ones," for Allah Almighty has commanded supplication and promised to answer it, and the aforementioned supplication is among the righteous supplications. Saying "Amen" to a righteous supplication is permissible, and the one saying "Amen" is one of the supplicants.