adding mapped sites to existing user

I installed the ‘multisite user manager’ plugin, but it’s not working properly or I’m missing some parameters.

I have 3 users and I want to add slowly more mapped domains, but I would like to add to 1 user the new sites. For this I have to access the edit mode of the mapped site, which is time consuming when you have many domains.
The multisite user manager supposely does this, but not in my case.

Can you please help to parameter it right?… or tell me another solution that with 1 click: I can add to an existing user one to several mapped domains in his account.

I searched in the plugin pages and checked several forums, but nothing realy points to a fast solution.
Is there something alike?

Thanks