Unfortunately, we do not have an option for that, but you can do the following:
1) Don't make your campaign public at first.
2) Change the number of entries people get for entering this campaign to
what you are going to give to your early adopters.
3) Notify early adopters separately about the campaign and ask them to
enter the campaign during a certain time period.
4) Then change the number of entries for entering the campaign to the higher amount.
5) Make your campaign available for everyone.