A plan excludes EE covered under a CBA. The same plan includes HCEs. Some HCEs are union members and want to participate in the union-sponsored plan.

How should these people be treated for 410(b) testing purposes?

Without having the benefit of the plan documents to review and study the relevant language, I would nevertheless think that the HCEs that are covered by the CBA are excluded from the non-union plan.

Excluded class definitions and provisions would be redundant if they only applied to employees that were otherwise ineligible under the plan. Thus, it would seem that the excluded class provisions override and exclude those employees that would be eligible, like the HCEs, but for the excluded class provision. Extending this rationale, the otherwise eligible HCEs are excluded from the non-union plan by virtue of being covered by the CBA.

As for these HCEs' participation in the union plan, as union members they are likely eligible--unless the union plan had a provision excluding HCEs.

For 410(b) testing purposes of the non-union plan, the excluded employees (including these HCEs) should be able to be taken into account as non-benefiting HCEs in the minimum coverage testing. That should help or make easier passing the minimum coverage requirement.