In a Profit Sharing Plan, if a participant elected out of plan participation at the date they became eligible to participate (meaning that they elect not to receive an allocation of any employer contribution), can they at some point in the future, reverse the election, and elect to participate?

Was the opt out based on the one-time irrevocable election provisions under the 401(k) plan regulations?

Yes it was. Employee received bad advice years ago, and periodically checks to see if there have been any changes that would allow him to "Opt" back in.

What part of "irrevocable" is causing questions?