I'm a senior PMM and the first marketing hire at a product-led-growth B2B startup. Do you have any advice on building out the marketing team as the company grows?

Be sure you understand and articulate *why* you're building out the team. We all want to grow the marketing team, but growing for the sake of growing can often cause more problems than it solves. To be effective at a small company with a small team, you need to grow strategically and focus on the most important initiatives. 

Put together a list of projects you'd like to tackle and other marketing-related inititatives. Then talk with your stakeholders in product, sales, and customer success to see what they would like to see from marketing. Work with those stakeholders to prioritize your list. Then approach your manager or executive with a hiring proposal for the next 12-18 months that addresses those priorities (e.g.: hire a GTM-focused PMM in November to accomplish these sales enablement initiatives, a Creative Director in January to address these needs, etc.).

It makes sense that the first marketing hire on a product-led organization is the Product Marketing lead :). I would say put your strategy together and depending on the priorities you will see what kind of marketing hires you need, vs perhaps going with external agencies for some marketing activities. You may want to keep the strategic items in-house like competitive intelligence, but anything related to marketing campaigns you could start with external agencies / contractors with the optionf or them to become full-time. It really depends on your audience and what you are trying to do next. For example, if you are targeting developers, my first hire there was a technical writer to clean up the product / API / SDK documentation. So sorry to say that the answer here is "it depends", but hopefully the context above can provide some pointers.

There are three capabilities you'll want to be sure are covered in forming a modern PLG marketing team. This is my take on their order:

1. Measurement: Make sure you have a Revenue Operations role in the company that will set up and maintain the data pipeline you'll need. At a smaller company, this will likely be a centralized Rev Ops role. You'll need to ensure they include Marketing considerations and data elements in their process. Ultimately, you'll need to make sure someone is helping you measure your impact on bookings and revenue.
2. Content Creation: You'll need someone who can create compelling, informative content for your target buyers. In a perfect world, this person will also have expertise in your domain, but that isn't an absolute requirement. If you already bring some of this skillset to the table, hire for the missing piece(s). For example, if you're a prolific writer, add design capability so that you have full coverage in content mediums.
3. Content Distribution: Once you have content, you need to get it in front of your buyers. This will look different depending on where your buyers spend time and your company's growth strategy. If your company doesn't subscribe to ad investment as a significant growth lever, you might double-down in organic channel expertise.