We do this in our team at Veracode! So we actually operate in a SCRUM/Agile fashion, with 2 week sprints. We point all of our work, and plan for an 80% capacity. This ensure we have time to drive ad-hoc requests and return immediate value to the business when they come up. In the event that we don't get ad-hoc requests for this sprint, and we find efficiencies, we bring in stories/work from the next sprint!
Ah, that's the million dollar question. At the beginning of each half we align with the leadership team which features and projects we're going to work on. This helps set expectations. Then I socialize with PMs what PMM is working on, which usually includes other projects besides feature launches. It's important for them to know what else you're working on to set expectations. Having said that, there's always adhoc requests and we either say no, and explain why we don't think supporting that request makes sense strategically, or provide some ammount of support.
So I use sprint planning for business. When it works well and we're compliant, it works beautifully. Here, we break our work into two week sprints and continously prune backlogs and review ad hoc requests. We also try to allocate 'white space" within the two week sprints for things that may pop up as needed. And we also have things like V2MOMs at Salesforce along with strategy / alignment decks that ensure we are marching towards the big uber goals.
We work with our PM team to create a quarterly roadmap. This helps us align with them on the major releases that are happening, discovery work we need to do, and align on key activities to influence growth.
We also then do a big marketing team-wide planning every quarter to ensure that, for example, those big product releases are also on Content & Demand Generation's calendar.
We then have a ticketing system where folks can input requests. We review these on a bi-weekly basis to see if someone has bandwidth to support.
It starts with aligning on common goals - what I find people get lost is in the "how" we get there. In business, we can all agree on goals that are like motherhood and apple pie - like revenue or cost savings. Hard to argue with those. Once you get aligned on that, then start with understanding what the recommended path is to get there. It could be what you're pitching or it could be something else. As long as you stay grounded in the shared goal, the rest is a lot easier, in my opinion.
Other than the things already mentioned throughout my answers (relationship building, offering to facilitate planning sessions, etc.), I would also try to show them your own marketing calendar/roadmap.
Share what you think your goals will be and ask to compare it to the rough product roadmap. Start by asking questions and sparking discussion. As you gain rapport and trust with your product teammates, they'll be more likely to bring you in at early stages, knowing that you are there to be a partner rather than an antagonist.
See my answer to a similar question on this page. You'll need to build trust and rapport with them and you do this by knowing your stuff, bringing unique insights, and having a well thought out perspective on key industry challenges. You keep overdelivering until your PM counterparts understand the value you bring to the table. If after a long period of serious investment on your part they still don't see the light, crumple your self-doubt into a tiny ball, toss it away, and find a new employer that values your PMM skills ;-)
You need to earn your seat at the table by repeatedly bringing fresh insights and a distinct point of view to the table. To influence product strategy and the roadmap you need to take a broader view of your market and the customers than your friends on the product management team. Invest in a deep understanding of your customers, your competitors, and the market at large. If you do this well your product counterparts will start to understand and appreciate the value you bring.
The PMM team at HackerOne reports in Marketing. PM's purpose in the universe is to build the right product by translating customer needs into products they can't live without. PMMs role is to translate these products into value propositions that move customers to action and help influence the product roadmap.