Hi,
I'm sure there's people who have been hired by a startup. I find the word 'startup' to be a bit vague though! Some people 'startup' alone or with one or two or a few more friends, ex work colleagues etc. Once running, often they're still referred to as 'startups' even though they've perhaps taken on more staff etc since !
The earlier in a startups history you go, I'd say perhaps it's less likely you'll get a chance to work there. It's only once they begin to expand and grow that you'll perhaps see 'were hiring' notices etc.
Every new company is different, so it's hard to say what the culture is like in terms of advancement and knowledge gaining etc.
I'm just about to launch my first 'startup' in the next 4-6 weeks for instance and I'm very excited! however, I must also point out that there's always another side to the coin. I've designed my startup so that I won't need to employ staff. If things progress very rapidly, then of course I will look at taking on staff, but for now and the short to medium term future, I've automated as much as I can so that I don't actually need anyone to help me. Previously in another family business, we expanded from 2 staff to around 30 within the space of around 6-10 months. Whilst there's excellent benefits to having staff etc, I've done the whole 'being an Employer' thing and all the hassles of it. I don't fancy going through all that again so early in my new startup. In short, there is a wave of thinking where small companies are looking to reduce costs as much as possible and staff is one of the most expensive single costs a business can have.
I don't mean to put a downer on your question, just be aware of it is what I'm trying to say. Other than that, get your CV/resume out to as many places as you can to see if your skills that you offer is a good match for a startup out there! Good luck!
Cheers, DtM! West London & Slough UK!