In API Connect, users can be onboarded in two ways:

• User Invitations: where a user is sent an invitation to either create and own an organization/catalog/space/consumer-org OR join as a member at one of these scopes.
• Administrative add of user accounts: where a user with admin privileges creates user accounts (with passwords) for other users and shares it with them.

In many cases, you might want to onboard multiple users at once or want to use the REST APIs for onboarding and script some of this. 

The following blog post explains how you can onboard users when user invitations are involved and how you can do it using REST APIs: 

https://medium.com/@nisha.narayanan/user-onboarding-in-api-connect-4cb747292cb4