How to organise roles and permissions of an organisation with a special project

[Marcello Verona]

I have to configure JIRA in the following way:
- all users in my organisation, identified by the domain @myorg.com, can access all projects.
- I then have a "BigProject" where external users with domains other than @myorg.com can log in with various roles.

I can invite external users to access JIRA, but I would like them to only be able to access the BigProject and by default end up there.

How should roles, groups and permissions be organised?

Thanks for your help!

Marcello

[Brant Schroeder]

@Marcello Verona welcome to the Atlassian community 

It depends on what you are using for authentication?  How are users from your domain authenticating into Jira?