This goes back several years, but the Steinernes Haus in the City Center would be a good choice. This is a 500 year old building (rebuilt after the war) and you sit at long wooden tables. The bier is excellent (what a surprise!) Food choices are traditional German but I remember ordering the Frankfurt Plate which came with enough food for at least 3 people! It was great but I could barely walk after attempting to finish. Seems like it would be a good choice for your crew. One annecdote. When I was there, they did not take plastic and that's all I was carrying except for a few measly DMs. Embarrassed, I explained to the waiter about my situation and said I'd leave them a credit card while I went to search for an ATM. They wouldn't take any "collateral" and trusted me to come back after finding cash. Yes I did! Here's a link:
http://www.steinernes-haus.de