Edit: looks like this is now legally permitted, https://tcf.org/content/report/virtual-labor-organizing/

”Due to a little-known, but far-reaching change made by the NLRB last year, virtual labor organizing by employees is now sanctioned by law in many situations and could possibly be transformative in the workplace ... NLRB recently reversed years of legal precedent that allowed employers to ban the use of workplace e-mail for organizing or communicating about the terms and conditions of work. Last year, in its decision in Purple Communications, Inc., the NLRB emphatically endorsed the right of employees to use workplace e-mail for such activities. It held that “employee use of email for statutorily protected communications on nonworking time must presumptively be permitted by employers who have chosen to give employees access to their email systems.” The NLRB added that: “Employees’ need to share information and opinions is particularly acute in the context of an initial organizing campaign.”