When must acquired employees enter plan after merger?

[Richard Anderson]

One company acquires the assets or stock of another company. Both the acquired and acquiring companies have plans. The two plans are merged. I understand that service must be granted in the merged plan for service with the acquired company.

May those employees from the acquired company that meet the eligibility requirements of the merged plan on the date of the merger be required to wait until the next entry date of the plan or must they enter the plan immediately?