There’s a good article on this subject written by Paul Graham — “Do things that don’t scale”: http://paulgraham.com/ds.html. We did many things in our startup that are well-explained in Paul’s article.
I work with a B2C startup founded in 2017, and at present, we’re on track to hit 4 million customers. For the first customers, I’d say the most important thing that we did is make people feel they are part of the mission. We established ways to let them know that they are part of the product development process — we created community groups, built a wishlist where they can submit feature requests, and made our roadmap more transparent. We built our beta program from the ground up and interacted with the customers like friends. We picked the most avid fans and formed a super fan group, and they’re still with us to date. We have regular reviews with them, and they are consulted whenever we feel that the decision will have a big impact on how they interacted with our products.
There’s a lot of things that you can do both on a strategic and tactical level, but the most important thing that you can do for the early customers is to make them feel heard. If you do this constantly and build a strong relationship with them, they will become more emotionally attached to your products and be your strongest advocates.