Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the following as you can (you can cut and paste this into your topic):
Nextcloud version (eg, 10.0.2): 12 beta
Operating system and version (eg, Ubuntu 16.04): Ubuntu 16.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.18
PHP version (eg, 5.6):PHP 7.0.17
Is this the first time you’ve seen this error?: Yes
Can you reliably replicate it? (If so, please outline steps): Yes
The issue you are facing:
None of my contacts or groups show up when I try to share any file or directory. What’s nuts about this error is that, not only can I see them all in my user list, but they’re all able to log on without a problem. When I type a user’s name in the sharing section, I get “No users or groups found for [username]”. I tried adding a new user and a new group as a test and neither of those showed up in the sharing section either. I tried updating from 11 stable to 12 beta in the hopes that that would fix it but it didn’t.
Is the sharing directory somehow looking in a different spot than my normal users/contacts? The attached screenshot shows you the problem in a nutshell.
The output of your Nextcloud log in Admin > Logging:
Shows no errors.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
The output of your Apache/nginx/system log in /var/log/____:
Log shows no errors.
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.