Connect to 365 and..

View default sharing settings
Get-Mailbox | ForEach-Object {Get-MailboxFolderPermission $_”:\calendar”} | Where {$_.User -like “Default”} | Select Identity, User, AccessRights

Add permissions to user for reception user to have Editor rights
Add-MailboxFolderPermission -Identity user1@domain.com:\calendar -user reception@gbdvlaw.com -AccessRights Editor
Add-MailboxFolderPermission -Identity user2@domain.com:\calendar -user reception@gbdvlaw.com -AccessRights Editor
Add-MailboxFolderPermission -Identity user3@domain.com:\calendar -user reception@gbdvlaw.com -AccessRights Editor
Add-MailboxFolderPermission -Identity user4@domain.com:\calendar -user reception@gbdvlaw.com -AccessRights Editor