should be MachineA/userA

For example, you have MachineA and MachineB
Even if you have same user name, the account is different, with MACHINEA/Name format.
For example, When you logon to B from A, and there is a user USERB there, you can use MACHINEB\USERB to login.
If you are using command line
the command is:
c:\>ner use * \\MACHINEB\SHAREB /u:MACHINEB\USERB
Then type password when asked.

Sometime, you might have problem resolve MACHINEB, then you use the machineB's IP address instead.