[c-nsp] Securing multicast

[selamat pagi]

We just introduces Pim-SM to our network.
Now we  encuntered a security issue:
2 PC connected to the same switch, 1 is allowed to join group X, the other
PC should not be able to receive data from group X.
The switch ports of those PC could be different every day and this issue
could concern several hundered users in  future.

Is there a way to authenticate multicast users?
Or how can such an issue be solved?

Any ideas highly appreciated

Cheers,keti