There are a few ways to automatically send a Google Doc to someone when they join a Slack channel, depending on what tools you are using and what type of workflow you have in place. Here are a few options:
Use Slackbot to send a message: You can use Slackbot, which is a built-in feature of Slack, to send a message to a new user when they join a channel. This message can include a link to the Google Doc.
Use a Slack app: There are several Slack apps available that allow you to automate certain tasks, such as sending messages or files. You can use one of these apps to automatically send the Google Doc to a new user when they join a channel.
Use a Google Apps Script: If you have a Google account, you can use Google Apps Script to create a script that automatically sends an email with a link to the Google Doc when a new user joins a Slack channel.
Use a Workflow Automation tool (like Zapier) to connect the two services
It is important to note that for some of these options, you may need some developer experience or IT support to get the integration up and running.