TheLostExploration

I wouldn't do any of what you're considering. A group email is incredibly impersonal and is not a good idea. It's also not your advisor's job to form your committee. Here's what I did: I thought about who I'd worked with and who would have good insight into my project and made a list of names. Then, I ran that list of names by my advisor. He vetoed one person*, recommended a person I didn't want because we didn't get along well, and said everyone else was fine. I then went around to their office hours and met with them for about 15 minutes. In those meetings, I explained my dissertation project in more detail (some already knew from having me in class or reading draft funding proposals) and asked if they'd be willing to serve on my committee. It went well and everyone I asked said yes.
 
*Note: I had no way of knowing he'd veto a person who was seemingly perfect for my project. But, he'd worked with them on committees before and was adamant about not doing it again. If this happens, do what your advisor says and skip that person.

I agree with rising_star: the general format should be 1) form a shortlist yourself, 2) discuss the list with your advisor and then 3) talk to each person individually and invite them to your committee. I think it's especially important to meet with professors and tell them about your research while inviting them to be on your committee. You want to show them that they will be a good contribution to your work, not just filling a seat. And if they are very busy, it is unlikely you will get a "yes" if you just asked them without any context or background. Be ready to explain what you are going to do in your dissertation!
A lot of people think the committee formation as a chore / just another piece of red tape to get through for the PhD. I'd disagree with this. I think the committee formation process is a really important and useful step because you are forming the "team" that is going to back you up throughout the rest of your degree! The committee is going to be the ones that write letters for you (in addition to your advisor), they're the ones that will let your advisor know if he/she is keeping you too long, they'll be the useful "second opinion" that you might want to seek in the future, and they'll help you expand your network. When I picked my committee, I looked for people that are in junior and senior positions, people that worked with theoretical and observational (experimental) approaches, and people that might ask me hard questions but would improve my work overall.

Have you asked older students in your program how they have approached it?  Usually in my department, you submit a 5-6 page prospectus to your potential committee members and then you meet with them each individually to talk about the project and see whether they are interested in being on your committee.  I am in the process of forming mine at the moment and the way that I am approaching it is to have 1-2 who are solid on the methods and 2-3 who are solid on some aspect of the content area that I am researching. (There is some overlap among the roles and everyone is familiar enough with the method.) If my research was really theory heavy, I would probably add someone who knew my theory really well but since I am doing policy research-- theory tends to not play as large of a role.  I know some of my cohort mates have added people who are good at offering emotional support when things get rough.  If that is important to you - that might be something you will want to think about.  
But I agree that you should form a shortlist yourself and run it by your chair before you approach anyone.  You want to make sure that your chair gets along with these people!  You might also want to have a couple of alternatives too in case someone is not able to be your committee for various reasons.  One of my cohort mates had to ask three people to secure his last committee member.  And make sure you read the rules set by either your department or school so your committee will get approved.

@TheLostExploration, don't view it as an interview because really, it's not. You're asking them to be on your committee after providing them with a summary of your project. They may ask a few questions about your plans but it really is nothing like an interview.