Some notes/tips for you:
- As admin, you can (batch) register many users through a .csv file, including the groups that each user should belong to. Useful to get started with many users when you know them... (for instance, educational scenarios).
- You might be interested in using the workspaces feature (available for 1.9.x or 1.10 as a Mod: Mod aulawiki). It has its own documentation, and its being used in production on some sites (such as http://edu.tikiwiki.org or others)
- In Tiki 1.10 a user chan chose to join a group at registration time (so that each user could chose his/her kind of usage...)
- There is not bundled shopping cart yet available for any Tiki version that I know of. Tiki 1.10 has a new "computed field" for trackers, and many tracker Plugins, which might help. Or Maybe using the Workflow feature, ... but you should have to do that yourself. However, the easier way for you is adding an external application to your server, and use it in conjunction with the Featured links feature...