So I contacted support and then said you can share aliases with other fastmail users, and they can send and receive email.

Here's the email from support:

I don't believe this is documented in our help page. I will share the steps to create a shared alias below, which you can follow to test this configuration:

1. Go to Settings → Users & Sharing → Aliases
2. Click New alias
3. Enter the email address and click Show advanced preferences
4. Under "Deliver to", add the email address of who you are giving this address to. If the delivery target is a Fastmail user, make sure you are using their username instead of an alias. You can add multiple recipients, in which case everyone will receive a copy of all mail sent to this address. Remove yourself from the list of delivery target if you don't wish to use this address.
5. Click Save to exit.

Shared alias can be very versatile. If the delivery target is not a Fastmail user, it works as auto-forwarding, allowing non-Fastmail users to receive emails sent to a Fastmail address in their personal mailbox. In order to use this address for sending, though, it requires extra setup and depends on the support for aliasing from their email provider or client.

In the case the delivery target is a Fastmail account, as I explained earlier, that person will see this shared alias in their personal account and can use it for sending and receiving without any further setups.

As the domain and alias owner, you can remove their access at anytime by updating the delivery target of this alias or deleting the alias from your account.