Employees of Subsidiary Eligible to Participate?

[Guest 409A newbie]

I'm dealing with a 401(k) plan sponsored by an entity with multiple wholly-owned subsidiaries. The subsidiaries have their own employees. The parent entity wants to let its subsidiaries' employees participate in the parent's 401(k) plan. Are subsidiaries' employees eligible to participate in a parent's plan? Can anyone point me to a code cite or reg? Thanks!

[K2retire]

IRC 414 talks about controlled groups, affiliated service groups and common control. You also have to look to the document. Some specify that all members of a controlled group are automatically included, others require that each individual company become an adopting employer.