One concern with what you said, "integrating my 'referral program' is when a user is within my website sign up process". <--- People are less prone to share right when they signup because they have no experience with your product or service... they just signed up.

I would recommend finding the most "sticky" areas for which there is customer delight. Here is some ideas:

- Maybe you have awesome tech. support that can fix things quickly and people love your company for that. What about on a ticket or chat close - depending on NPS score you ask them to share?

-Taking publishing a website for an example: What about right after they publish since they have had the experience with your company to properly evaluate whether they like your service enough to share with their audience/influence?

I have more ideas - I am in game for a call, if you are:)