How to share users tables in 2 separate installations

I have a wordpress site already set up and want in install a 2nd wordpress site in a sub-domain of the first installation. I want to be able to allow users to login to both sites using the same username and password for both sites. I also want them to stay logged in if they are logged into the main site and go to the site on the sub-domain.

How do I achieve this?

I have tried following some tutorials I found and when installing the 2nd installation on the subdomain and keep getting this error during installation:
“Catchable fatal error: Object of class WP_Error could not be converted to string”