Hi @JAD Admin
Thank you for posting your question in Microsoft Q&A.
We understand that you are considering adding another account to a Distribution List group in your tenant. Based on your description, it appears that you are using a personal or business account that is not part of the same tenant.
You could follow these steps:
Add contact in your tenant:
Go to the Microsoft 365 Admin Center.
In the left pane, navigate to Users -> Contacts.
Select Add a contact, then enter the account information.
Click Add to save the contact.
Wait approximately 10 minutes for the changes to sync.
Add a contact to a Distribution List:
Go to Team&Groups in -> Activeteam&Groups
Select the Distribution List you want to update.
Click Members -> View all and manage members -> Add members.
Choose the Mail Contact you just created.
Please feel free to correct me if I misunderstood your request. If you have any additional concerns, feel free to comment below. I would be more than happy to assist.
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