If you know which workstation belongs to which human, you can create a logon trigger that logs all instances of the specific shared user(s) over a period of time, and you can review the table to see which workstations are logging in. I would put the logging table in a stable place like
msdb or a utility database if you have one.
CREATE TABLE dbo.ThatLoginLog
EventDate datetime2 NOT NULL DEFAULT sysdatetime(),
Now, the logon trigger:
CREATE TRIGGER LogTheLogin
ON ALL SERVER
WITH EXECUTE AS 'sa'
IF ORIGINAL_LOGIN() = N'that login name'
INSERT msdb.dbo.ThatLoginLog(HostName, IPAddress)
SELECT HOST_NAME(), client_net_address
WHERE session_id = @@SPID;
Then, periodically, or as soon as you think you've given enough time in your business cycle for everyone to have logged in at least once:
SELECT HostName, IPAddress, COUNT(*), MAX(EventDate)
GROUP BY HostName, IPAddress;