Y Combinator startups are excited about all the value the incubator is creatingY Combinator welcomed five new partners to its elite cabal today and said goodbye to one.

Paul Graham revealed in a blog post that Wufoo’s founder Kevin Hale will join YC as a full partner. Wufoo makes an online form building application. The startup participated in one of YC’s first classes in 2006 and was acquired by SurveyMonkey in 2011. Hale spearheaded user experience and design for Wufoo and will join existing partner Garry Tan as a design mentor, which Graham said “partly reflects the increasing importance of design in startups.”

Michael Seibel, Steve Huffman, Dalton Caldwell and Andrew Mason will join the accelerator as part-time partners. Seibel cofounded Socialcam which went through the Winter 2012 YC class and quickly became the most popular application on Facebook with 54.7 million users. It was acquired by Autodesk in 2012 for $60 million. Huffman cofounded popular travel app Hipmunk and Reddit, which both went through Y Combinator.

Not all the new partners are YC alums. Caldwell cofounded App.net and Imeem and Mason cofounded Groupon and held the position of CEO before the board ousted him in February. Harj Taggar, who is currently a full time partner, is becoming a part-time partner so he can travel and then work on new startup.

Y Combinator now has ten full partners and eight part-time partners.

The accelerator program is known as the most prestigious in the world, a “startup machine” that churns out “billion-dollar ideas.” It was founded in 2005 and provides a three month bootcamp, seed funding, and mentorship which culminates in a demo day where the entrepreneurs pitch to a room filled with wealthy and influential investors.

YC has fostered some of today’s most well-known startups like Reddit and AirBnB and achieved some impressive exits. Its summer 2012 class was the biggest yet with 84 startups, but this number caused a “bottleneck” that led the partners to dramatically decrease the size of the next class.

“The reason we accepted fewer applications was that in summer 2012 we grew too fast,” Graham said in a blog post. “We had 66 companies in winter 2012, and that was fine, but for some reason more things than usual broke when we jumped from 66 to 84.”

I covered the summer 2012 demo day and spoke with many of the founders who said the main drawback to the large size was less access to the partners. Part of the value of participating in YC is being able to learn with, brainstorm, and collaborate with experienced and successful mentors who can help guide the entrepreneurs and products towards eventual success. Decreasing the class size helps open up this pipeline, as does adding more partners.