You can only give access rights to someone if they are a member of your organization. If the person you want to give access rights to is not a member of your organization yet, let them buy a ticket in your webshop. They do not have to finish the payment. The person can only login to the system if they have an account. So when the person buys the ticket they should select the option "I want to create a Dizizid account" if they do not have an account yet.
After the person pressed the button "Place order" you are able to find the person in the member management section. There you can see that the person has a member account.
Click on the tab "Roles", there you can assign a role to the person. Once the role has been assigned the person can login with their own personal account into the system.