When a user is created in LDC Via, they do not have access to any databases until they either import their own database (in which case they, and any super users can access the database) or until a super user grants them access to one or more databases.
The process for granting users access to databases is administered in the System -> Groups screen. In there you have two tabs on the left hand side of the page. The Groups tab shows lists of groups in your organisation, you can select a group and add users to it.
The right-hand tab shows Users. For a user you can perform a few tasks:
Define new variations on their Notes name to be used in document level security
Data migrated from Domino will have "Notes style" usernames in Readers and Authors fields. For example, "CN=Fred Bloggs/O=ACME". The user account in LDC Via uses email address as the key identifier. When you mark a field as being readers and authors, LDC Via looks for matching user account based on either their email address, any groups that the user belongs to, or any alternative names that are defined on the user account.
Add the user to different groups to be used in document level security
Document level security also makes use of Groups. If a group name is included in a readers or authors field, then any members of that group will be given the appropriate access to the document. Read more about groups.
Allow the user to access different databases
From here you can add or remove access to databases for a user.
In all cases, these operations can be performed using the REST API: