One of the things that can be hard, when choosing to come out to parents, is the fact that you might feel like you have to educate them about gender issues, both on a general level and in terms of your own identity; this can make a process that might already feel overwhelming or stressful even harder to manage. Letting an organization that's dedicated to this sort of education do some of the work for you can take some of that weight off of your shoulders.
Also, it's helpful for parents to have their own source of support in handling a child's gender identity or transition. Of course, you're going to be the best expert in your own identity and what support you specifically need from your family and loved ones, but it might be a big help for everyone involved if you can connect them to some of these organizations, just so you don't have to do all of the heavy lifting with regards to getting everyone else on board.
This is all US-centric, I'm afraid, simply because that was the scope of the conference. Some of these are national organizations; others are based in and serve smaller areas, like a city or a state, but provide enough helpful information on their websites to be useful to a wider population.
Gender Spectrum: This is a Bay-Area-based group that provides a wide range of support to parents and caregivers of trans and gender-variant youth, including information about medical, legal, and education options. They provide local in-person support, but they have a lot of information online and provide support by phone as well. Having been to the Family Conference, I would certainly recommend it to parents who want to get a lot of in-depth information on how to access and provide support.
PFLAG: Not all PFLAG chapter groups are equally educated about trans issues, but at the top level it’s an extremely supportive group; this is a great resource for parents who need their own support. There are chapters across the US and the central organization has a lot of good basic, introductory information online. This could be a great resource especially for parents who might feel like they need extra support.
Trans Youth Family Allies: This site has information and resources for trans youth and parents; the parent resources include basic introductory information, help navigating medical transition and transition at school, and some tips on how to be an ally and advocate for trans youth. Some of their resources are available in Spanish as well as English.
Trans Active: An education and advocacy group for trans youth and their families based out of Portland, Oregon. They provide educational trainings for schools, hospitals, and other organizations, have several in-person support groups, run a binder-exchange program, and have an active mailing list. They are doing work currently to expand their in-person services outside of the Portland/Pacific NW area.
Trans Youth Education and Support: An advocacy group based in Colorado; they've done a lot of work on safer school programs and have also worked on a PFLAG advisory committee to ensure that their trans support is as helpful as possible. They have meetings in-state but also have plenty of resources on their website as well.
Trans Youth Equality Foundation: An advocacy and support group based in Maine; they provide workshops, training, and support services for youth and families. Their website also has information on legal and medical issues, as well as social transition at school.