Easy365Manager supports editing multiple users in one go. To assign an Office 365 mailbox to multiple users simply select the users, right click and select properties and go to the Office 365 tab. Assign an Exchange Online license and click OK:
If the users are not in the same OU, temporarily move them into the same OU, assign the mailbox and move the users back to individual OU’s.
Alternatively, use an LDAP query to search them out, select the users you want to change and perform the above steps.