Consider these interpretive rules:
“[T]he plan administrator of an employee benefit plan subject to the provisions of part 1 [of subtitle B] of title I shall furnish a copy of the summary plan description . . . to each participant covered under the plan (as defined in § 2510.3-3(d))[.]”
29 C.F.R. § 2520.104b-2(a) https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XXV/subchapter-C/part-2520/section-2520.104b-2
“An individual becomes a participant covered under an employee pension plan” no later than “[t]he date designated by the plan as the date on which the individual has satisfied the plan’s age and service requirements for participation[.]”
29 C.F.R. § 2510.3-3(d)(1)(ii)(A)(2) https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XXV/subchapter-B/part-2510/section-2510.3-3#p-2510.3-3(d)
If a statute is ambiguous, a court defers to the agency’s notice-and-comment rule if the rule’s interpretation is permissible. Chevron, U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (June 25, 1984).