How Do Lobbying Groups Work?

[theBruiser500]

So say restaurants want to make a lobbying group to represent all the restaurant interests to the politicians. They get together and start contributing a little money from each company to the lobbying group. What would stop one company in the restaurant industry from not paying dues, but accepting the benefits that would come from a lobbying group for all restaurants? And, for that reason it seems like every company would just not join.

Another question about lobbying groups. How does a lobbying group actually apply pressure to a politician to adopt the lobbying groups platform?