I set up a matrix server. Each OpenClaw agent gets its own bot user and room. I can @tag agents, invite them to rooms, and query them. I have one agent that has admin access to matrix and can create additional agents. WhatsApp kind of half works but it's tedious and flaky and you end up chatting with yourself unless you have a lot of spare phone numbers to create bot users. Slack flat out doesn't work because they've locked it down. Other things are available. But Matrix does the job and is straightforward to integrate with OpenClaw. Also great if you want to use this with multiple users.