you can share it in the sense that you can walk it over to his house and let him use it anytime you want. i take mine with me all over the place. once all the accounts are set-up, so long as it has a wifi connection then you can access all the stuff you pay for.
for both of you to use it at the same time, you will each want your own roku device. the good news is that you can use the same accounts for netflix, amazon and other services by sharing passwords. so he could have his own roku but use your netflix and other accounts as well. netflix adds a buck a month to allow multiple devices but it is a cheap price to pay for 2 people to use the account at the same time.
this is what i would do myself is share passwords but each have your own roku device. the hdmi stick is only $40 on sale and can even be cheaper if you get a refurbished one online.