What do you think is the most effective way to invite beta users to a web app?

Have you already got an online community with members? If so this could be a method in which you could get people involved in BETA testing, another good way is to post of forums like these asking if anyone would be interested in beta testing. Their are many other ways but you've just got to come up with somthing creative

It's always hard when your a less known community to get people to help out but once you get big you'll have no problems at all, Im sure Dani will tell you that

I started onelurv from practically from zero. Not well-known, no big community, zero budget.

Months went by. It grew slowly, but it grew nonetheless. I dunno what to feel about it.

But I'm hungry for growth. So I need all the marketing tricks I can get.

It's an interesting concept your website, a good idea would be to get in with the proffesionals in that area of the internet, read my article here. Basically if you start talking to them they will be able to give you some helpful info. I hope that will help you.