Basic interview process is 3 parts. First, a phone call with HR. Next a phone interview with the hiring manager. Purpose of the phone interview is determine if they want to fly you out to SF for onsite interviews. If you make the onsite interviews, expect to have multiple rounds of interviews with the entire team. You will also be expected to give a presentation to a faux customer scenario. You will be given details to prepare for presentation prior to the onsite interviews. Yammer hires by team meaning if one person on the team doesn't like you, you're out. Don't expect a warm reception if you're older than the CEO either. Expect lots of attitude and egos.

I had two phone interviews with the company, and it seemed to me the interviews for this position centered around the ability to articulate the value of the Yammer product to the prospective client. Being knowledgeable about the product and reading up on the company seems to be key. Refrain from comparing the product to facebook or twitter.