sharing data

I have multisites working. Now I think want to share data. The reason I may want to share data is that I would like to market products to all the users of these sites. Note that each site has its own brand and each have unique features (different data, different modules, etc.). There are only a few things that I would like to share.

These may be either:

1) Users so that I can send everyone emails from one core site,
2) or a shop catalog that sells a common set of products.