Allowing users who are not part of Azure AD to access Azure SQL managed instance.

Here are the concise steps to achieve this.

1. Login to Azure portal and open Active Directory.
2. Create a guest user and invite to your organization.
3. Once the user has accepted this invitation he/she will show up in the Azure AD users blade.
4. Create a group in the Azure AD and add that user to that group.
5. Connect to the managed instance using SSMS and create the user group with name as same as in Azure AD

Using the following query.
USE master
GO
CREATE LOGIN [TestGroup] FROM EXTERNAL PROVIDER
GO
6. Give permissions to that group on the SQL managed instance using SQL as you would in SSMS.

Link is below.

https://docs.microsoft.com/en-us/azure/sql-database/sql-database-managed-instance-aad-security-tutorial