Assign Permissions on the Virtual Machine Level in VI: ESX

I'm hoping this is a really simple question. We just got an ESX 3.5 server. I'm using the VI Client 2.5. I'm trying to be able to set permissions on the Virtual Machine Level.

e.g. User1 can start/stop  Virtual Machine A, but not Virtual Machine B.

User2 can start/stop Virtual Machine B, but not Virtual Machine A.

In working through the client I see how to specify if a user can start/stop a Virtual machine on a host level (e.g. All Virtual machines on the given host), but I can't seem to figure out how to do that at the Virtual Machine level.

I've searched the DOCs and I must just not be using the right search terms.

Thanks for your help!