User Role at Site Level

Since we can create multiple site in Neos
Can we do something like when user “A” can access only site “” and not others(Defined many site)

Yes, you can do that with the permissions layer and Policy.yaml. Have read about the permissions layer here

is there no option in Neos portal itself where i could assign easily?
I went through the link which you have shared.
But I am afraid where to link the specific site with customer role.

Would you please give in details too

Not at the moment