Shared mailboxes are used when multiple people need access to the same mailbox, such as a company information or support email address, reception desk, or other function that might be shared by multiple people. Users with permissions to the group mailbox can send as or send on behalf of the mailbox email address.
Step 1: Sign in with your Office 365 admin account.
Step 2: In the Admin center, on the left pane, click Groups drop-down and select Shared mailboxes.
Step 3: Click Add a shared mailbox button on top of the page.
Step 4: Enter the name of the shared mailbox and click Save changes.
Step 5: You can now add members to the shared mailbox by clicking Add members to this mailbox option or add later by clicking Edit in the Members section while viewing the particular mailbox from the Shared mailboxes list.
Step 6: Now click the Add Members button and check the check-boxes respective to the accounts and click Save.