There's a pretty simple litmus test I use for advisory board members: would I be worse off without their advice?
That may sound ridiculously simple, but really ask yourself if your advisors are going to help you come up with solutions to the really big startup problems, like overcoming extreme technical hurdles, finding investors and helping to mentor you as a leader. Any member of a startup team, be it employee, investor or advisor, must provide strategic value. If they don't, then don't bring them on board.
To answer your specific questions:
No, there isn't much value in a small startup, unless you really are leaning on this person for operational mentoring. Or unless they are a superstar in your industry and will actively bring investors or customers to the table.
It's as formal as you choose to make it. My advice would be to set out specific goals and timeframes for these advisors. That way both parties know what is expected, and there is no question of the value being exchanged (i.e. your equity for their effort). These might be things like providing 3 quality investor introductions per quarter, or meeting with your technology team twice per month for an architecture review.
Find out what networking events they attend and bump into them. Spend some time getting to know them in an informal setting, then setup a formal meeting. Or, use LinkedIn, someone you know probably knows them. Get a warm introduction from a friendly 3rd party. Also remember that you are also gauging their merit, don't put yourself into the position of a supplicant, as that is a sure fire way to get pushed around.
Group meetings will never get you anywhere, as everyone will have strong opinions and it will devolve into a mom and apple pie meeting with no clear outcome. As I said above in #2, set out specific goals and timeframes, then follow-up individually on each task. Make sure you're also building the relationship and not just asking for work product.
What value would your accountant bring to the table? Were they a CFO of a Fortune 50 company? If it's just your accountant and they aren't giving you strategic advice that you couldn't pick up from any startup blog, then it's not appropriate.