Private-equity investors often use strategies to order carefully rights and relations regarding investees, and often design these to keep investees separate from one another and from each investor.
Beyond carefully defining and allocating each capital interest, profits interest, or income interest, an investor might assert that it is not “a trade or business”, and so is not to be combined with any other investor nor any investee.
Don’t get involved in trying to sort out what is or isn’t a § 414(b)-(c)-(m)-(n)-(o) employer. Get your client, preferably with its lawyers and accountants, to instruct you on what you are to assume in performing your services.
This is not advice to anyone.