Apart from collecting groups/users from external systems, do we have option to create users and groups manually in VIA?
VIA L&G is not meant to be the authoritative source of user/group information and hence cannot/should not be used to store such information.
Said this (and not recommended), no one will stop you from creating custom tables in VIA L&G's Oracle database for such purposes. You could even throw in a register user form, a database collector and a connector to take user inputs, create user information and then collect.
Retrieving data ...