Hi there @laylo,
Welcome to the Community! 
I'm sorry for the current limitations with the Trello integration. At the moment, it doesn't support adding or inviting members to a board. We've heard similar feedback from other users and our developers are aware of this interest.
Good news is, I've added you to the list of users interested in this feature. This does a few things:
- It brings the issue to the attention of our developers.
- It helps us gauge the demand for this feature.
- It allows us to notify you directly if this feature becomes available in the future.
While we can't guarantee that all feature requests will be implemented, your feedback is incredibly valuable in helping us improve our platform. If this feature does get developed, we'll be sure to let you know!
I understand that this isn't the ideal situation. Please don't hesitate to ask in the Community if you have any other questions or concerns. We're here to help! 
Hi! Is there a way to be notified once this feature becomes available? :)
Yup, @Jillyn - thanks for letting us know! I’ve got you added and we’ll be in touch if this gets implemented. 
I also need this functionality. @christina.d
Hi @doughenrique93,
Thanks for letting us know! I have added you as another interested member to the open feature request. We will keep you in the loop when this feature gets implemented.
As far as I know, for this to work with Zapier, does Trello need to add this action to their REST API? @ken.a
Hey folks,
I’m happy to share that an Add Member to Board action has finally been added to the Trello app! 

Thanks so much for your patience on this, we’re excited for you to test it out! If you need any help with it or have further questions at all, don’t hesitate to open a new topic here or connect with our Support team.
In the meantime, happy Zapping! 