I’m sure there is.
The Discourse Zapier integration doesn’t have an Invite command either. This topic is actually explaining how to get around that issue by using a Zapier action step that allows you to make an API call to Discourse. I’m fairly sure you can achieve the same thing on make.com by using their Make an API Call action: https://www.make.com/en/integrations/discourse.
You might find useful details in this topic’s OP about how to set that up. If you have questions about getting it to work, it would be best to ask them here: How to Connect Discourse with Hundreds of Apps Using Integromat. (Integromat was renamed to Make.com).
@JammyDodger the topic that I linked to should probably be renamed and edited to make it clear that it refers to Make.com not Integromat.