Friendship is friendship, business is business, make sure you have a written agreement in place just as you would for any business arrangement, it prevents potential misunderstandings on both sides and may save a friendship (or litigation!) if things go wrong.
As someone else mentioned, IB is good for trading multiple accounts, they offer Friends & Family Advisor and a versatile PAMM (Percent Allocation Management Module) or LAMM (Lot Allocation Management Module) which makes it easier trading and accounting for multiple accounts. Each friend has their own individual IB account so they are responsible for their own tax liabilities as far as I know.
The worst thing I found about trading other people's money is they drive you mad wanting to know how their account is doing! Have an accounting period (monthly/quarterly/annually) so they leave you alone to trade, nothing worse than dreading a call from a concerned client when you're in the middle of a losing streak!
And most importantly don't forget to make them absolutely fully aware of the risk involved.
On top of all that, trading other people's money isn't suitable for everyone and requires additional discipline and psychology because it's easy to feel pressurized to make money for them, especially if you've exaggerated a little when telling them how lucrative trading can be.
