Who is permitted to make investment elections when one 401(k) plan is being merged into another existing 401(k) plan?

More information needed.

For Example: Company A has 50 employees who are all participants in their 401(k) plan. These employees have participant directed accounts. Company B has 350 employees who are all participants in the Co. B 401(k) plan, which has participant directed accounts.

Co. B employees have made elections. Company A plan merges into the B plan and the former participants in A plan must make elections in the B plan election options.

What are the restrictions on Plan B participants?

If your question refers to the participants who are already in B, the merger has no effect on them, so (presumably) there is no reason for them to make new elections. However, because mergers often involve "fund mapping", it may be that fund changes also occur at the merger date, thus impacting all participants.