The best approach for building referrals into your sign-up process is going to depend heavily on what it is your users are signing up for.

Is it something performance-based, like a fitness or language learning app, where the user can challenge a friend to compete with them for a higher score? Is it a service the user is signing up for, that he would recommend to a friend if he has a positive experience?

Start by identifying the motivation a user would have for referring a friend. (They're likely not going to share it on social media just because you ask them to, without giving them any incentive.) Then figure out where within the sign-up process it makes the most sense to ask them to refer a friend. You may also find that the best time to ask might be outside of the sign-up process, further along in the user's experience. Whatever you choose, make sure you craft your message carefully to align the user's incentives (to get through the sign-up process quickly but not miss anything that might add value for them) with your ask for their referral.