The history files (.bash_history for bash, .sh_history for ksh) are just command history files to make it easier to reuse the command. They aren't really designed for auditing which is what you want to do. You can do "strings" against such files for users (in their home directories) to get the commands but they won't have time stamps associated. Also since this history files roll over time they won't have all commands. It is a good place to look if you suspect a specific command was issued and want to find out who did it but it doesn't really prove it because you can't tell WHEN they did it. (Usually you don't have to prove it - people are generally shocked enough that you figured out it was them.)
To really do full auditing you need to plan for and turn on system accounting. This does quite a bit of logging so you'd need to plan for having a lot of space for that.
This page may help (I found it with a Google search):