Sorry for the late reply. What you are wanting to do can probably be achieved, but not with the Discourse Zapier integration that is outlined here: Use the Zapier Discourse Zap Templates. Instead of using that, you will need to point a Discourse Webhook to Zapier using the method that is outlined here: Trigger a Zapier task with Discourse Webhooks. Detailed documentation about how to create a Salesforce lead via Zapier is here: Create a Salesforce lead via Zapier when a user signs up on Discourse. The process for adding members to your Active Campaign list should be similar.
There is an issue with the method that I am suggesting though. It relies on the Discourse User Event webhook. That webhook is triggered every time a user logs in, logs out, confirms their email, is created, approved or updated. This means that each time the webhook is triggered for a user will be counted as a task on Zapier. Depending on how busy your site is, that could cause you to use up all of your Zapier plan’s available tasks fairly quickly. It would be great if the User Event webhook could be filtered on Discourse so that it was only fired when a user is first created. That would be useful for integrating with services like Active Campaign and Salesforce.