You can send an email to both prospects and supporters but they have to be separate messages. 

In NationBuilder, the people in your database are categorized into two buckets: Prospects (people who have not yet opted-in to receiving contact from you) and Supporters (people who have actively opted-in to receiving contact from you). Here is a little more information on how these two categories are defined.

The reason people are split into Prospects and Supporters is because it's important to communicate with these two groups of people in different ways. Prospects may be people that you think would like to hear from you, but since they haven't actively indicated interest, you should be sparing and brief in your communications with them, with the ultimate goal to have them opt-in to future communications. Since Supporters have already indicated that they want to hear from you, you can be more flexible in your approach to contacting them.

Because of this, NationBuilder requires that you send separate email blasts to your Prospects and your Supporters. This means that you cannot send one email blast to everyone in your database- you have to send at least two blasts, one to each group. Here's a little more information about how to set up an email blast.

Again, we always encourage that you target your email blasts to Supporters and Prospects differently, as if you send information to Prospects that some of them aren't keen to receive, they may mark you as spam, which has the potential to drastically inhibit your email from getting out to a wider subset of your recipients.