Reach out to people 1-on-1 who fit your customer profile. I did a research sprint and got lots of people to test out my MVP after following up with responses to my polls about SXSW. You can read more about the research sprint technique I used here: What I Learned About Your SXSW Experience
(and it's totally worth the extra time to do it this way!). Similarly, @kevinrose
started a Facebook group and added everyone there for his new meditation app
. He provides constant updates and gets continuous engagement through there. Lots of people offered to help with designing the app!