You do not need a separate WordPress username or password. Open any site from My Instances and select WP Admin.
WordPress opens in a new tab, already signed in as an administrator.
Why this is better than a password
There is no WordPress password to share, lose or have guessed. Access follows your account, so removing someone from your account removes their way in.
If you need a real WordPress user
Some plugins, and some people you work with, need an actual WordPress login. Open the site, go to Advanced → Users, and manage the site's WordPress users there.
