What does it mean when an operating agreement issues Class B interests for investor introductions on top of a placement fee?

Two different kinds of pay are in front of you, and they come from different pots.

A placement fee is cash, paid once, usually at closing, calculated on the equity raised. On a deal raising 5m, a 1 percent placement fee is 50k. It comes out of the total equity before the money goes to work, so it reduces what the deal has to buy and improve the property with. Your 100k isn't invoiced separately, your share of that cost is baked into everyone's returns.

The Class B interest is a share of the promote, also called carried interest. Promote is the sponsor's cut of profits after investors get their preferred return, so it only pays if the deal makes money. Giving a slice of it to whoever brought the equity in is common.

Strictly, a co-sponsor is someone who is a member of the sponsor entity with real duties and real economic risk. Loosely, the market calls anyone who brought a chunk of the equity a co-sponsor, including people who did nothing but make introductions. Those are very different roles wearing the same word, so ask which one this is.

Two things to request in writing. A full sources and uses showing every fee as a percentage of equity, so you can see the total load, and a plain answer on who sends your quarterly statements and your K-1. Whether a person can be paid for bringing in investors at all is a securities law question, and only a securities lawyer can tell you if a given arrangement is fine.