how can I add a collaborator?

You can add collaborators to your form for them to build it with you by editing your form and on the upper-right part of the form builder you will see there the Add collaborators button:

Once you have the link, you can share it with your friends. When they open it, it should open your form and they will be able to modify it as much as you do.

Note: It would be best if your colleagues shall have an account. This way, you will be able to identify who are currently modifying the form as they will appear on the upper right part of your form when you are editing it.

Please check this guide as well to know more about form collaboration: https://www.jotform.com/help/419-Understanding-Form-Collaboration

Thanks.