Greenfield, a former Child, Youth and Family social worker, wanted to set up a service that promoted the welfare of teenagers and supported them in becoming "functional and positive members of society".
Her own adolescence was trying.
At 11, Greenfield emigrated from Samoa to New Zealand with her family.
Not long after, she found out the woman she had known as a sister was actually her biological mother.
It made her angry, she says, and that anger influenced her behaviour in the years following the revelation.
Seeing the distress her conduct was causing her grandmother, who Greenfield calls her "real" mother, and becoming a parent herself at 17, motivated a change.
Siobhan also sees the world differently since becoming a mother 10 months ago.
Plenty of support and some tough love from her Yes to Youth social workers have led her to mend strained relationships and use her abilities in a part-time administration role at the organisation's base in Rangiora St, Castlecliff.
For Siobhan and Greenfield, difficulties in the lives of adult family members had a detrimental influence.
Greenfield says this is a common problem for many of the young people Yes to Youth supports.
"That chaos trickles down," she says.
At the same time, and on an even higher level, policy makers ignore young people.
She says they need to be more aware of the impact their regulations will have on youth - now and in the future.
Yes to Youth is about pushing a message of hope.
It is founded in Greenfield's passion to care about and help young people.
Helen Hall, the organisation's business manager, has helped Greenfield realise her vision, which involves a constant, frustrating and, at times, demoralising search for funding.
Hall appreciates the fact there is a high demand for grants, although it doesn't make it any easier when an application that has taken hours and hours to complete is declined.
But the overriding success of Yes to Youth and its clients is a good antidote.
When it began operating in March 2007, Yes to Youth supported eight families - it currently assists 41.
Alongside these one-on-one support services, it operates a community technology centre, youth centre and group programmes for teenage parents.
Hall, who grew up in Castlecliff, says seeing the results of the organisation's work in an ex-client who is employed or in training and "standing on their own two feet" is hugely rewarding and motivates the significant amount of pro-bono work Yes to Youth's staff engage in.
Even so, the organisation will require $460,000 to operate this year.
As a non-government organisation, any services it provides the public sector are only partially funded.
Hall says that is the standard practice.
The Government expects the rest of the funding - normally about 25 per cent - to be provided by the community.
No socio-economic group is immune to the problems associated with difficult teenagers, according to Greenfield.
She says Yes to Youth's clients come from "all walks of life", impoverished to wealthy.
And her message to parents who are struggling to cope is that they are not alone.
"Normally people know, for example, that when your child is sick, you put them to bed or take them to the doctor.
"But for some people, their lives are in such chaos that they don't know what to do - it's just a big mess."
Often in these cases, a Yes to Youth social worker will act as a surrogate mother or father, which keeps the teenager accountable and gives their parents a break.
To contact Yes to Youth, visit y2y.org.nz or phone 06-3443399.