Hi Nicola,
Even though it is not possible from Arc to see who is using Arc. You can use Pulse to see all users who used Arc to connect to TM1.
From the Pulse Live Monitor, there is a Context column showing you with which tool (Arc, TM1Web, Architect, Workspace…) the users are accessing TM1:
All this information is stored in the Pulse database.
You can use the default H2 console to query the Session table as below:
Select DISTINCT UserName From Session Where Context = 'Arc'
With Pulse 6, you can use the Pulse Explorer to retrieve this information without having to write a SQL query:
More information about how to connect to the Pulse database can be found here:
I hope this help,
Cheers,
Vincent