Startup Join wants to modernize the back office for an industry that’s everywhere, but maybe not top of mind, especially when it comes to project management software: Commercial construction. The company has raised a $4 million seed round, co-led by Signalfire and Building Ventures and including participation by existing investor Bolt. The startup’s core product is a collaborative decision-making platform designed to facilitate more effective working relationships between everyone involved in the preconstruction phase of a building project, including owners, contractors, designers, tradespeople and suppliers.