Enable the Friends List?
This will allow users to easily access a list of other people they have added as a friend, and quickly send them a Direct Message or see their online status -- even if they are online in another chat.
You can disable the Friends List on the Community plan and higher.
Enable the Ignore List?
This will allow users to click someone else's name and click Ignore.
Ignoring a user will prevent their messages from showing up, and prevent any Direct Messages from going through.
You can disable the Ignore List on the Pro plan and higher.