Step 1: Login into portal.office365.com
Sign in with your Login ID and Password.
Step 2: When entering account click on Admin as shown into the picture.
Step 3: In the Office 365 admin center, choose the Active users option
Step 4: Click on the Username and click on the Mail.
Step 5: Click on the Mailbox Permission.
Step 6: Click Edit against Read and manage feature as shown in the figure.
Step 7: then click on Add permission.
Step 8: Click + Add Permissions to add the user mailboxes whom you want to assign this permission and select the User.
Step 9: After click on the Save button now you can see that the user mailboxes are assigned permissions.