How do you partner with product/product strategy early before the roadmap is finalized and what role does your PMM team play specifically in the early phases?

This can totally vary team to team, company to company, product to product. Like you said, you want to do all of this early, the question just becomes how early and what kind of value PMM can drive the most for that particular product or roadmap.

Sometimes product marketing should be the first team to touch a new product bet. When there’s an adjacent space or market that you’re eager to break into but don’t have clarity or conviction about the problem space or product needed to succeed, product marketing can do the upstream research about the market, competitive dynamics, buyer criteria, decision process, and more to pick the right wedge for your investment, giving the product team a clear idea of what’s different or uniquely valuable enough to win over buyers. Other times, when there’s a stronger foundation of knowledge in the space or when the product is a known entity, product marketing’s role becomes more important in the downstream phase of that product launch or roadmapping process – crafting the position you want to occupy, the differentiated narrative you can amplify to the world, and equipping your customer-facing teams to win.

Either way, getting involved early means being proactive around establishing credibility and competence in GTM strategy, being the expert on the market, your customers, your product, and your competition.

The way I think about this is that PM owns the roadmap. What I mean by that is that, none of my PMs--past or present--have sat down and brainstormed about roadmap with PMM. So in the moments where I've felt that we have an area that is unadressed within our roadmap or that by not adding an item to our roadmap at a certain time puts us at risk of a market moment, the onus has been on PMM to drive the steps necessary to win over PM as a stakeholder around what the future product strategy should be.

To do this, I'd suggest you look at timing. When is the right time to field research so that you can collect insights in time for it to actually shape a product teams plans? Is there a market moment -- such as a big company event -- that is at a fixed time in the annual calendar where you can showcase market insights that would inform PM to adjust roadmap in order to make a bigger splash/inform PM that without that change in roadmap their feature wont be showcased? 

I think you also need to look at your PM/PMM relationship as a whole. My current team has built a reputation with our product stakeholders about being upstream in the product strategy conversation and that we will bring insights about the market and the user to our product teams to help them. Other teams at my current company tend to be more downstream and PM/Eng make the decisions on where the product needs to go, and PMM builds campaigns around those features. If I were on a different team, the expectation to influence roadmap wouldnt be the same, nor would the willingness from PM to think about what PMM is suggesting be as strong. 

As for my specific team, we tend to do a lot of research. We partner with our PMs and User Researchers to map out what lists of questions we need solved to help build a great product, and split research among PMM and UXR depending on the type of work. PMM also does a lot of desk research to help inform PMs of market trends. When I was on B2B teams, we'd bring in analysts like Gartner or Forrester to bring 3p research to the table. I tend to be somewhat inquisitive around product data so I like to also look at any internal usage trends that may paint a picture. Across that combination of 1P/2P/3P research, I try to think about what decisions I would make if I were a PM on the product and what bits of information I might find valuable. I then compile into a lit review/summary and bring back to product to ask questions and brainstorm. 

From here, I tend to then let Product build the roadmap and if something feels left out I'll bring it up and have conversations about it in our syncs, but ultimately my goal is to ensure that our team (PM, PMM, Eng, UXR, etc.) are all on the same page around what inputs are needed to make product decisions. This POV of focusing on being an influencer, while bringing insights back to the team, tends to keep us with a seat at the table as products go from zero to 1. 

Roadmaps are a living, breathing plan. This should be a continual conversation with product (and sales, and cs, and...) to make sure you always know what is happening and changing. And, I've never seen a roadmap that's truly "final." It's technology. "Stuff happens."

But, there are multipltae stages when you should have both input and visibility into the plan:

1. Early/vison setting. What's the long-term target we're trying to hit, and what are the key investments/stages on the way? Here you're getting input into the order of operations as well as the stories you'll want to tell along the way
2. Concrete, mid-term. This is the 6-12 month view (depending on product timelines). Now you're engaging around launch planning and customer comms. What are the big themes you want to build around? What content marketing investments do you want to make to define the market, key differentiators, etc. Big point of input: What can you package/land together to tell a better story? 
3. Delivery time, short term. This isn't so much input as awareness. What's new, what's changing, get into the gory details.