Looks like this question got the most upvotes so let me start with it and try and give as much context as I can.
At Google we had the following heuristic for marketing which was:
1.) Know the user
2.) Know the magic
3.) Connect the two
It informed our marketing overall, but especially our positioning and messaging. You asked about both and I would actually separate the two.
For positioning it's always a good idea to start with the product and engineering teams. They are typically the closest to the vision of the product--who it's for/what it does/how does it do it i.e. the "magic". I often use the metaphor that if the product is a movie, PM and Eng will typically cook up the plot, while it's product marketing's job to deliver the narrative.
Sales is also a good stakeholder for positioning. They'll help you narrow down the "user" details, especially in enterprise where they spend more time with customers than anyone else in the organization. So it really behooves you to spend as much time with sales as you can. Done right, product marketing is sale's best friend.
So at big companies, prod, eng and sales are typically good places to start for positioning while at smaller companies, like startups, you may want to involve the founder and leadership team early as they have most of the datapoints. But I'd keep the stakeholder list pretty tight overall when it comes to positioning otherwise you'll tend to swirl and it's really important to nail positioning first, because messaging is the by product.
Per the Google heuristic above, positioning covers the user and the magic. Connecting the two is where messaging comes in, and that's where you should open up the aperture when it comes to stakeholder management IMO. This is where Demand Gen, Field, AR, PR or even support stakeholders comes in.
The reason is simple. As the product marketer, people will and should treat you as the SME when it comes to the product positioning but will likely want to help shape what messaging looks like at the last mile, whether that's on the web, email copy, print ads, events and activations, or when talking to the press and analysts. After all, they own those channels, are directly responsible for their performance and know what typically plays well and performs--or not. The worst thing you can do is go against the grain and be precious about a turn of phrase or a cool tagline.
At Salesforce, we had a campaign where with partners where we thought we could riff of the expression "being on cloud nine". Well that worked well in NORAM, but didn't make sense in other markets where the expression was meaningless when translated.
So when it comes to positoning, be ruthless and direct about who has input as you don't want to swirl on that or compromise. But for messaging, take the inputs, and to paraphrase Bruce Lee "be like water". While you shouldn't let the substance change, you want your message to resonate.