have done in the past, not leading a housegroup as such atm. It can take as much or as little time as you like. We (DH and I) always used toask other members to lead bits of it to share the responsibility around. A good starter is to ask each membrr to share a bit of their story, who they are and how they became a Christian, how God has woked in them etc. means you get to know each other at the start. If you are leading you need to be willing to talk first though. THis can take several weeks to get through all the members.
one model of smallgroup that I found helpful is the ws:
welcome - tea and cake, smalltalk, and also an intro type question that everyone can answer, something really simple
worship - sung or otherwise, or listen to a cd etc
word - some aspect of the Bible - either a discussion about that weeks sermon, a Bible study, meditation, whatever - some churches suggest questions for smallgroups to ponder.
You can buy Bible study guides, or you may find there is someone in your group good at leading Bible study...this is the bit we often asked others to do the most
witness - reaching out to others - this can be through prayer, maybe praying for your local community, or news stories or whatever, or planning and doing low key events - coffee, pub quiz, child care swaps whatever
wonders - expecting God to turn up, giving Him time to speak - when praying for instance.
is that any help...happy to chat more