Two options are available for a company that has adopted a plan that affects tax liability and obligates the company to pay benefits. The first is if they have a Board of Directors who has approved adoption or amendment of the plan through board action, they are not technically required to have an adoption page, but this is not applicable to smaller companies so an officer must approve the adoption of the plan or amendments to an existing plan.
Typically during audits, the adoption page is requested. In the context of the company committing itself to providing benefits and taking pre-tax deductions, an officer of the company is required to agree to it. The adoption page can be excluded for the documents distributed to employees, but it should be signed, dated, and maintained with the original document with other company records.
Large companies rarely use adoption pages as the Board of Directors has authorized it and the signatures are on file via the board minutes.