Email servers, electricity, the time spent setting it all up, the server room that stores the machines, the IT department that maintains the machines and keeps things running, the client hardware that the employee interacts with...
That said, even if it is detectible, I think that employees should be allowed to drain some small amount of a company's resources to organize. I am allowed to use company resources for emails like "who wants to go for beers after work?" why should I not be allowed to send "who wants to go organize collective action after work?"