Show user ID in admin/people list?
It's pretty easy if you're able to install the Administration Views module.

It replaces the default admin/people page with a view, which you can easily customise to add the user id as another column in the table.

If you're not able to use that module there are ways to do it in code, but it's a bit fiddly. I did roughly the same thing a couple of months ago (before I'd come across Admin Views) so let me know if you want me to dig the code out and post it here.