arp -a in a Windows Server 2008 R2 prompt, will the returned entries cover connections from all users logged on to the machine or only the current?
The arp cache is a table containing all the arp entries the machine knows about. It's not collected or segmented in a user-specific fashion.
Be aware, however, that dynamic arp entries expire fairly quickly, so your arp table might not even be a good representation of MAC addresses the computer has collected, just a good representation of the past couple of minutes' worth of network activity. (From the link above):
Dynamic ARP cache entries
These entries are added and deleted automatically during normal use of TCP/IP sessions with remote computers. Dynamic entries age and expire from the cache if not reused within 2 minutes.