Can I use autocomplete to add users in a webform (they might be existing contacts)?
Yes, it can be done with webforms. 
Select a 2nd contact in the CiviCRM tab, check Existing Contact and add any other fields that you want to expose on the form (including the custom info). Click Save Settings.

Go to the Webform tab and click on the field that was created for the Existing Contact. There you can set the autocomplete option for the Form Widget (in the Display area):


Take note of the permissions setting in the Filters area as well so you don't end up exposing a lot of personal information unintentionally.