Which is the privilege to enable intenal conversation?

Summary:

HR Help desk Redwood

I have custom role for SME and Agent. Standard role for the Administrator.

Content (please ensure you mask any confidential information):

Whit administrator user I'm able to start an internal conversation and search people but when I try to start one with an agent or SME the search doesn't work.

Which is the privilege to add to custom roles that enable this function?

(plus: How can I restrict the list from all employees to only agent or SME?)