One site per user

Hi,

I am building a multisite with subdomains. It will look like this:

Mainsite: mydomain.com
Site 1: sub1.mydomain.com
Site 2: sub2.mydomain.com
Site 3: sub3.mydomain.com

Every site is a site for training employees. The trainings will be similar, but every subsite is for another company.

What I want is that users that are registered at site 1 only have acces to site 1, so they can’t login at site 2 and/or site 3. At this moment if a user is registered he has access to all sites, including the main site. I don’t want employees of company 1 (on site 1) to have acces or login to the site of another company.

I have been searching for days now and I get very confused if this is possible or not. And if I need a plugin or not. I am using Ultimate Member for the login procedure (it works really nice) and am curious if the feature I need is something I can arrange with UM.

(The strange thing is that if I look at all users via the network settings, I can see a differentiation on the acces of the site, some are appointed to the main, other to the subsite, but when I am testing they all can login in both sides)