Signals Notebook: Is there a way to have some sort of group membership selection during the login, such that, post-login, a user would only see those items allowed by the selected membership?

Not exactly during the login, but users might be able to achieve the result by using e.g. search filters and attributes. Please refer to our System Configuration Guide > Group Associations. We also provide examples for some use cases in System Security Policy > Policy Examples chapter of the same guide.