All financial institutions involved in selling or marketing a new issue of debt or equity but not necessarily participating in the underwriting consortium.
Taobiz explains Selling Group
In many cases, the selling group is simply the underwriting group and does not include anyone who is not responsible for underwriting a portion of the new issue. The reason why the selling group may include only the underwriters is that these underwriters are unwilling to let competitors who are not bearing any of the risk to participate in the selling process.
Alternatively, the selling group may include a number of parties whose sole focus is to sell an allotment of the newly issued securities on behalf of the underwriting group. In this case, the non-underwriting sellers are sold the securities at a markup, but still at a discount from the expected market price for the security. These non-underwriting sellers make money at the transaction by the spread between their price and the market price, if it's higher than what they paid for the security.