Hi Laila,
Thanks for your clarification. Now, I understand the situation is:
All departments are using 1 library. Each department may have several shared mailbox (not one shared mailbox for each department). When you want to set up alert for an item, you may need to let several department members to get the notification. However, as one department has several shared mailbox. It’s difficult to add all shared mail box for those departments. You want to create a group which include all shared mailbox in that department. When you create alert, you just need add the group for that department rather than input all shared mailbox one by one.
If the above assumption is correct, the following are my understanding for the scenario:
To let users get the updates notification, we need to set up alert for the item. To set up the alert, we need to use a group which can also be used as mail box. SharePoint Group cannot be used as mail box. We could choose to create Office 365 Group or mail-enabled security group. These two kinds of groups also can add shared mailbox as members. I did a test, added user1 to one shared mailbox, added this shared mailbox to an Office 365 group and mail-enabled security group, set alert for a folder with Office 365 Group and mail-enabled security group, made changes to the folder. However, user 1 didn’t receive any notification for changes in the folder. Then, I set up alert for two files (this is more similar to your requirement), one alert is related to mail-enabled security group. The other is related to Office 365 group. User 1 received the notification which is related to mail-enabled security group.
You may confused, let me explain in this way:
User 1 is a member in a shared mailbox which is called Shared2, Shared 2 is added to an Office 365 group and an mail-enabled security group. Open OWA for user 1 and add Shared2 to folders (open shared mail box). Set up alert for a folder with Office 365 Group and mail-enabled security group. No notifications. Set up alert for two files with these two groups. User 1 can see the notification for the alert related to mail-enabled security group.
So, for your requirement, you could try creating mail-enabled security group in Office 365 admin portal for each department, and then add related shared mailbox as members, set alert for individual files with mail-enabled security groups. Check if users in the department can receive notifications for file changes.
Best regards,
Tim