I would like to have the @mentions restricted for community users where they can only see users from their account or internal team members from my company. Is this possible?
You need to set the default external sharing settings for the User Object to private. If this is set to private you can use "Portal User Vibility" from the Sharing Settings to allow user to chat with other users from their account. Internal users need to be added to the community (by profile or permission set) to enable community users to chat with them.