It's tricky to provide very concrete ideas without seeing the product, but here are some generic suggestions:

1. Make them understand the value of sharing
It might seem obvious to you, but people have a million things going on at any given time so you need to get them thinking about this. Instead of just saying something like "share your site with friends" tell them about the benefits of doing so. I don't have a whole lot of information about your product, but it seems like the benefit would be getting more leads? In that case you could use language such as "show off your new site to your friends and get more leads".

2. Make it easy to share
Remove as many hurdles as necessary so it becomes really easy for people to share their site. For example add share buttons with pre-written texts, provide 1-click sharing if possible (e.g. if they oauth'ed their social account), etc.

3. Show them the benefits/results of sharing
You want to create a positive feedback loop where the user gets 'rewarded' for sharing their site. If they know it works, they will keep doing it. So for example you could show website analytics (clickthrough counts, etc).

Without knowing much about your product.. If the users friends aren't likely to be leads consider some kind of basic "recommend this person" feature and then make it clear that sharing your page is an important way to build up your recommendations and look better to prospective leads. Maybe in order to recommend someone it's just lightweight social auth, but then you can lead the recommender through the process of setting up their own page if they are interested and try to create a nice feedback loop there.