What can I use to Audit Cisco Users? Which user has performed which commands?

You can use cisco ACS server which provides AAA.

You can use AAA Server (Authentication, Authorization and Accounting).

Audit Logs allow you to track changes that have been made to your Cisco router

Specifying Audit File Parameters.

Router (Config) # audit filesize128  (Valid values from32KB to128 KB)

Router (Config) # audit interval120  (Valid values ranges120 sec to3600 sec)

To view Audit file

Router # show audit

Show audit displays audit file information such as the timestamp and what are is being hashed

To view detailed information use following command

Router # debug audit