Main resource: Add or remove Azure role assignments for external guest users using the Azure portal
- External identities settings
https://portal.azure.com/#blade/Microsoft_AAD_IAM/CompanyRelationshipsMenuBlade/Settings

more information:
https://docs.microsoft.com/en-us/azure/active-directory/b2b/delegate-invitations
2) Create a guest user
Azure guest user portal

3) Add a role
Azure resource groups portal
4) Accept Invitation
Connect you on you main accoutn and navigate to: https://portal.azure.com/resource.onmicrosoft.com
info: Add B2B collaboration guest users without an invitation link or email