Add User to shared mailbox:
Add-MailboxPermission -Identity Shared@domain.com -AccessRights FullAccess -InheritanceType All -AutoMapping:$true -User User@domain.com
Add Send as permissions:
Add-RecipientPermission -Identity Shared@domain.com -AccessRights SendAs -Confirm:$false -Trustee User@domain.com
Remove User from shared mailbox:
Remove-MailboxPermission -Identity Shared@domain.com -AccessRights FullAccess -Confirm:$false -User User@domain.com
Remove Send as permissions:
Remove-RecipientPermission -Identity Shared@domain.com -AccessRights SendAs -Confirm:$false -Trustee User@domain.com