The other person is at a different company. They will not join your Slack. You will not join theirs. Somebody suggests a "shared channel," which is a third workspace nobody wanted, or a Google Doc that becomes a graveyard of unresolved comments. The work still needs a decision.
You do not need a room you both inhabit. You need a message they can open, an ask they can answer, and a rule about who is allowed into whose inbox.
Shared Slack is a room, not a letter
A workspace assumes the same employer, the same retention policy, the same joke about the same incident. Across a company line those assumptions fail. Counsel should not live in your #general. A vendor should not see your other vendors. A board member should not have to install another client to say yes or no.
Email already solved the transport. What it did not solve is composition: a cold reader at another firm still gets a wall of context written for people who were in the room, or a paste from an assistant that sounds like a briefing and is not one.
The message has to stand alone
Write as if they will open it on Monday with no Slack history and no memory of your last call. Name the decision. Close the option list. Mark the premises you are guessing. Put the ask where a tired reader will find it.
That is a briefing, not a channel. It travels as email and a web page. The recipient does not need an assistant, an account, or your stack. If they later connect one, they can pull the same package. If they never do, they still reply from the link.
Writing to someone who is not on RelayLink is the ordinary path for a first send. Receiving is free. Nothing to install.
Consent is the substitute for a shared roof
Once the other person has an activated account, a stranger should not be able to drop packages into their assistant's inbox. That is the consent gate: an accepted contact pair, or the send is refused. The refusal looks the same whether they have an account or not, so asking is not a way to discover who is on the product.
That is stricter than Slack's "if you have the channel, you can ping." It is the right stricter. Across companies, access to someone's assistant is not a courtesy. It is standing permission to put words in front of a model that will act on them.
Two assistants can still talk after that permission exists. They do not have to be the same product. Claude on your side and ChatGPT on theirs is a normal pair. The package is what they share, not a workspace.
What you stop doing
You stop creating a third Slack so two people can finish one question. You stop CC'ing a thread of people who cannot answer, hoping the one who can will notice. You stop forwarding a chat because it is the only artifact you have.
You write one person, one ask, and you wait for the reply. If they are not a contact yet, you send the email. If they are, you already have the pair. Either way, nobody had to join a room to do the work.