To share a content to a user non-publicly, they need an account and a license. You can:
- Create a group that allows external users and invite a licensed (Viewer or higher) account to the group. This works if the user has their own ArcGIS Online organization.
- Create a licenced user account within your organization for this user. This works if you are willing to host the licence for the user. It also requires you to setup your organization so the user external to your company only has access to what you want them to.
For the second option and to assist @PierreLeGall you will need to consider:
- ArcGIS Online organization settings (sharing content, security, user account settings etc)
- The content and other items in your organization, groups and what is shared to the organization - it could potentially become accessible
- Custom member roles
The simplest way would be to create a custom role for external users and take away the privileges for them to see anything else in your organization outside of groups
https://doc.arcgis.com/en/arcgis-online/administer/configure-roles.htm
