I think consortium commissions are a wonderful way to fund the creation of a new work. Every one has been a bit different for me, but I’ve narrowed in on a general approach that seems to work pretty well. All of the items below come from my personal experience, but I’m all ears if you have any suggestions or want to do things a little differently. Also, I’m happy to help at any point in the process if you have any questions or concerns.

– The consortium agreement is signed once both parties are happy with the details. The deadline for members to join the consortium is set for a few months after that.
– The lead commissioner(s) recruit members. Each member pays their part of the commission fee, to the lead commissioner.
– The maximum number of members is reached (by a pre-determined deadline) and half of the commission fee is paid to the composer. The other half is held by the lead commissioner.
– The final piece is delivered on or before the final deadline and the last half of the commission fee is paid to the composer.
I have a standard commission agreement for consortium commissions that is very similar to the one I use for regular commissions. You’ll have plenty of time to look this over and ask questions before signing.
With a regular one-person/ensemble commission, half of the commission fee is due upon signing the agreement and second half is due once the piece is delivered. However, I’ve done things a few different ways with consortium commissions. I’ve found that the best way to do this is to set the first payment deadline to coincide with the deadline for new consortium members to join. The final payment and delivery deadline is the same as a standard commission. Lastly, getting the full portion of each member’s fee to the lead commissioner when they join saves a lot of headache down the road. That way, you’re not trying to get a second payment from each person when the final deadline approaches.
Typically, the lead commissioner has exclusive rights to the official world premiere and consortium members have exclusive rights to perform the piece for one year after the premiere date. In most consortiums that I’ve been a part of, the lead commissioner chips in a larger amount of money than the members. (Typically 2 or 3X the amount of the other members) Aside from world premiere rights, the lead commissioner is listed as such in the score and his or her name is at the top of the list of commissioners.
I’ve generally taken a very passive role in organizing the consortium and recruiting members. However, I’m happy to take a more active role if you prefer. I’ve also found that having a firm deadline for when members can join makes things easier and adds a sense of urgency that helps with recruiting. 
I’ve never experienced any difficulty when it comes to recruiting enough members for a consortium. That’s likely because networks among musicians are so close-knit and an email or social media post from the lead commissioner usually seems to be enough. However, I can also help to recruit members if any difficulty arises.
I’ve been commissioned by consortiums as large as 33 and as small as 8. With a larger consortium, it can be easier to recruit members because the fee to join is smaller. However, a smaller consortium can give members a sense of belonging to something that is more exclusive. It really just depends upon what you feel would be best. On a side note…  A couple of past lead commissioners have tried to make the total number of consortium members flexible, but I’ve found that can lead to complications down the road. 
Most lead commissioners that I’ve worked with have set up a simple web page for potential members. This allows them to easily see what it’s all about and join the consortium with a few clicks. This page typically includes info about the details of the commissioned piece, number of spots left in the consortium, and a paypal link to pay the fee to join. I’d be happy to build a page for this if you’d like.
I like keeping performers involved in the composition process. Maintaining a private blog for commissions has proven to be very effective. Here’s a link to one I maintained for a consortium commission a few years ago. In addition to keeping consortium members engaged during the composition process, it’s a great way for me pose questions about extended techniques, notation, etc and quickly get a variety of perspectives.