how assign common user to multisite

Hi,

There is situation of 1000 users within Multisite
100 per batch users will be part of their subsite as group_1.mainsite.com

so 100 group user will have major access to their group subsite.

Now situation is this 10 Editor need to access of all group subsites
means we need to give common access to few subsites , but do not want
to create duplicate users in every site

how to achieve that ?

and how we can allow / disallow - one subsite user to chat with specific subsites or not.

Regards,
ajay