Hi Diego,
There are 2 ways to try what want to do.
1. Sign up for a free-tier sandbox account (completely FREE). Setup your sandbox account (add users, add phone numbers etc.) and create an app and test the app in the sandbox environment with the users you created in your sandbox.
2. Ask your customer (the one who has an existing RingCentral account) to create an app and let you have access to their sandbox environment for developing and testing the app.
If you want to develop an app for more than 1 RingCentral accounts to use your app, perhaps, you should start with the option 1 above, create a public app with 3-legged authentication, test it and graduate it to production. This will allow any RingCentral users from different accounts to login your app and use it.
Alternatively, if you develop an app for just 1 RingCentral account to use your app, then you can start with the option 1 above, develop and test the app. But you have to follow the option 2, asking your customer to create an app (same settings as the app in your sandbox), give you the app and user credentials to test that app and finally graduate it to the production environment so that they can use the app with their production account.
Hope this help.
