Enable group postings using organic groups on my drupal site

I've got a drupal6-based site which I'm ready to deploy except for one issue. I would like to be able to let a few users on my site make postings to groups of other users. The feature must support the following: - If a posting is made to a few of my groups, only users belonging to those groups should be able to see them - The postings may contain attachments - The attachments must exist in a place which can only be accessed if you are a signed-in user with rights to those files because of the group(s) to which you belong - The site administrator should be able to see who has accessed the various private postings - The groups must not be searchable, and my user list must also be not searchable - Group administrators must be able to add users, remove users, create postings, edit postings, and delete postings Optional but nice-to-have features: - The site should send email to the members of a group when a posting is made to that group - Users within each group should be able to edit some, but not all, of the postings specific to that group I would like to use an off-the-shelf plugin or few plugins, rather than develop custom code. I think Organic Groups can do what I'm looking for. What will be required is to set up OG (Organic Groups) or the other plugins, and also to document to me exactly how a group administrator can accomplish the tasks above In order to be considered for this contract, you must: 1. Be able to point me to at least 1 web site which you built using Drupal (v5 or v6) which supports similar functionality 2. Have positive feedback from other projects within the past 6 months 3a. If you plan to use OG, please explain briefly what limitations might exist on my site 3b. If you plan to use something other than OG, please tell me what plugins you will use 4. Be willing to give me daily updates, if this task is going to take you more than 2-3 days. I will escrow the money when we begin the work. I will not release any money until the task is completed. Please don't ask me to do so.