In Revolution, there are no "web users," just users so they’ll all go in the modx_users table. I don’t know offhand if the table fields are different, but you should be able to export them to a CSV file (which might need modification if the fields are different) and then import them to the modx_users table.
To control their access, you’ll have to assign them to user groups and create ACL entries for the groups for the various contexts. I don’t know of an easy way to do that but maybe someone else will have a better idea.
BTW, you don’t need to use contexts for this (although you certainly can). You can just use resource groups and tie them to the user groups with ACL entries. To do it with contexts, you’ll need to "switch" contexts in order for the users to see the various documents. Only one front-end context can be active at any time and if the users will be moving back and forth between contexts, it may be easier just to use resource groups.
This may help you understand the new security process:
http://bobsguides.com/revolution-permissions.html
