If you’re only dealing with individually boards then I would individually invite the people to the board or only share the link to that board with the group. 

For example:

Set up the board > Share > Individually invite via email or setup a share link and give those people permission to edit 

Everyone else would have the “no access” option when sharing and then they wouldn’t be able to see of edit the board. 

This is can work with boards in the project. But it we have boards in the main space without projects, than everyone who have an access for any project can see this boards.

@Chocoorchid - There is no user group management in Miro. As @Joanne Richardson pointed out, Projects are you only option - and you will need a paid-type plan in order to use Projects.

But it we have boards in the main space without projects, than everyone who have an access for any project can see this boards.

Correct. You would need to modify each board’s Share settings to hide the boards that are not in a Project - you would set the “Team” access to “No access”.

I have outlined all of this, and a strategy for managing groups of users in Projects, in the following post:

https://community.miro.com/ask-the-community-45/how-does-teams-and-accounts-works-4785