Celebrity does not have and never has had a welcome aboard lunch. I suspect that if you have that many people going it is basically a private function arranged by your Travel Agent or whoever is running the group. Again, Celebrity has never had a welcome aboard lunch.
Comparing your first post on this thread with your last post, something doesn't make sense.
"We are first time cruisers on Celebrity .The people we are traveling with are repeat cruisers and will be invited to the Welcome Aboard lunch . Is there any way that we can join them ? "
"Everyone in the group we are traveling with (103 people) are return Celebrity cruisers and all are going to the Welcome Aboard Lunch . "
Your first time cruises but yet everyone is repeat cruisers, doesn't make much sense.