When adding External Guests to a SharePoint Online site in the browser you might be prompted an error "Couldn't add user@external.ie as a member."
In order to resolve that, navigate to the group as per below:
1.
2.
3. Click in the 'go to Outlook' link:
4.
It is important to note that the External Users must have a Microsoft account in order to be invited to a SharePoint site / Teams. Once they are added to the site, they will receive an invitation by e-mail which must be accepted. If the invitation is not accepted the user will be prompted a Access Denied message stating that the no record for the user was found in the environment.
Also another requirement is that the site has to be configured to be shared externally, some organizations don't allow external sharing and you should contact your administrator to clarify if that is configured.
Useful resources:
- How to register for a Microsoft Account
