Earlier this year, the IRS released proposed regulations under section 414A.
Proposed 1.414A-1(d)(4)(ii) says that "For this purpose, the number of employees that the employer normally employs for a taxable year is determined using the rules of Q&A-5 of § 54.4980B-2 of this chapter." https://www.federalregister.gov/d/2025-00501/p-188
54.4980B-2 Q&A-5 defines "employer" by reference to Q&A-2 of the same section.
Q&A-2(a) defines employer as " (1) A person for whom services are performed; (2) Any other person that is a member of a group described in section 414(b), (c), (m), or (o) that includes a person described in paragraph (a)(1) of this Q&A-2; and (3) Any successor of a person described in paragraph (a)(1) or (2) of this Q&A-2."
For an employer who chooses to rely upon the proposed regulations in their current form, I would conclude that they are required to include other members of their related groups when determining if they meet the exemption of 414A(c)(4)(B).