Do you have any tips for developing a Product Marketing function in an org where the Product Management team hasn't been long established?

In the early stages, as teams are starting to get built - trust and dividing responsibilites is the biggest factor in ensuring alignment, especially between PMM's and PM's. 

How do you build trust and clearly divide responsibilities? Communicate and clarify your end goals. 

Align on goals - Ownership from both teams is necessary. Product Marketers are customer and market facing, while PM is developer and product facing. PMM's must ensure product adoption is critical and their highest priority. Marketers must align product packaging and messaging with market demands and PMs must align requirements. Teams must have shared goals, product deliverables should be married to marketing deliverables that speak to the customers desired response. Having one end goal, that is strategic and not tactical, allows both parties to work together to drive the products success forward. 

KPIs - Divide KPIs between both teams whether its customer or product oriented. Product managers care more about usage and customer satisfaction while product marketers drive growth and retention. Create a clear set of KPIs and get buy-in from internal stakeholders and partners. Product marketers must always have metrics attached to their launches, its the only way to ensure there is accountability and more importantly, showing the PM team you too are in it with them. 

Market research - provide visibilty into market conditions, sales, customer feedback, and industry research is prevelant. Consistently share this information with PMs in order to help shape the products trajectory and success. 

This is a really tough one. First and foremost, it's important to separate PM and PMM. They need to be parallel, not dependent, organizations to effectively function. You need them to have the freedom to call BS on each other, work through differing opinions, and emerge with a stronger view of the market. 

I'd recommend starting with the problem to be solved. Is it improving overall GTM effectiveness (opportunity creation, pipeline, win rates), or accessing a new market or vertical? What are the overall priorities of the business that demand gen or product are unable to address on their own? 

Once you identify the problem, then the justification for the function (or a hire) flows a little more naturally. You can also quickly draft up objectives and success measures based on the problem you're trying to solve.