Ask HN: How much of a concern are bots for a smallish community app?

I'm building a small app with community and social features. Most of the features require an email/password signin, and I'm wondering how much of an issue bots are? It'd be easy for someone to generate a lot of dummy accounts using temporary email addresses. I could add captchas to the signup page, but that feels like a bad UX choice, especially if I'm just being paranoid.

Anyone else use a different method? Admin tools, etc?

I should also note that the app involves small amounts of money getting exchanged among users, so there might be more of a motivation for spammers than a basic social site.