Does Corda node support the concept of an organization and users in the organization? The ORG user must have visibility to transactions of the node. And is it possible to add users within this Organization who can be part of selected transactions?
Yes, you can do it with Corda. I see two ways:
Use the Account library. Accounts are "logical" entities, i.e. subset of the node's vault. An ORG can be the Corda node and the users can be accounts owned by the node, which has complete visibility of the transactions. You can add as many accounts as you want. Note that an account only has public keys, the private keys are owned by the Corda node owner of the accounts. The flows are effectively ran by the Corda node, not by the Account themselves who are just effectively states that only have a name and a set of public keys (but not a X.509 identity, so they are not registered to the network. Only the Corda node is). A typical use case of this is a Group Company X (i.e the Corda node) who owns some Subsidiary Companies (its accounts). More info: https://training.corda.net/libraries/accounts-lib/ and https://github.com/corda/accounts/blob/master/docs.md
Use the Business Network Membership: https://docs.corda.net/docs/corda-os/4.8/business-network-membership.html. In this case you have different Corda nodes connected to the same network, and a subset of these node share a "logical" network, which is made at application level in which you can set the roles and memberships. In pratice, this is a cordapp shared between them where there are states that identify an organization and its rules.
These two above are ways to effectively create "organizations". In Corda then you also have the Observer parties, who are parties that just want to be notified of some transactions without effectively be part of them.