I didn't see an option to enable it (but I could have missed it). I do know that a lot of large forums don't allow people to do that because it uses a significant amount of server resources when pulling all that info. I'll let @NYGuns answer for sure though.
In the meantime, if you have a particular username that you want to check, I can do it for you.
If you know the beginning of their name, you can type it in the search on the members list, on the right side. It's not perfect, but it's something. For example, if you type "ni" it populates NirvanaFan and Nicys Firearm Instruction
Found the option, talked with the guys, and enabled it. You should see it show up as "Registered Members" when you go to the members page (or mouse over the arrow next to the Members button.
Edited to add... We're going to leave this on for the foreseeable future. However, If it starts killing the servers resources because we get a lot of members, we reserve the right to turn it off.