This feature is available on the following plans: Grow, Scale, Growth (legacy), Business (legacy)

You can do this from Settings > General Settings by first toggling on the option to allow Direct Messaging:

Clicking Manage Rules will show you a pop-up with this rule: "Admin in any group can message everyone". This rule cannot be removed, to prevent Admins from getting locked out of Direct Messages.

From here, you can set up different rules, following this logic:

[Role] in [Group] Can message/Can't message [Group(s) or User(s)]
 

Outcomes are determined from top to bottom: Your most important rules should be towards the top of the list. You can re-arrange rules by dragging and dropping them from the dots on the left.

Once you're happy with your setup, click Save:

Messaging logic

1 on 1 DM chats

• If either member lacks permission to message the other, the chat is locked for both (no one-way chats).

Group DM chats

• If a member cannot message any other member in a group DM, they cannot message in the group and their message box will be disabled.
• If at least 2 members in the group DM chat have permission to message each other (via the rules), they can message in the group. Only the members that have restrictions will have a disabled message box.

Admin overrides

• Admins can create group DMs that "break" the rules.
• Admins can always reply in any chat they're a part of.

How to test rules

There is a rules tester where you can check the outcomes of the rules you're currently seeing. 

Click the ↔ button between the sender and recipient to switch places:

Examples of what you can set up

• Members can't message each other, but they can message Admins and Moderators
• Free members can't message each other, but Paid members can
• Members in an Automated Group can use DMs between themselves, but members outside of the group cannot