You cannot file Head of House hold unless you have a closely related person (such as a child) that you support as your dependent and you pay more than 1/2 the entire costs of maintaining the household.
As far as your other question, unless you have a formal rental agreement with the friends, you are probably just sharing expenses. If you need something formal, they should first find out if they are allowed to sub-lease in their rental agreement.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
