Hi Bryan,
you don't have to take care for releasing the connection. This happens automatically within NCo runtime. By setting the destination parameter RfcConfigParameters.ConnectionIdleTimeout (IDLE_TIMEOUT when using app.config) how long a connection is kept open for re-use before being closed. With RfcConfigParameters.IdleCheckTime you can control how often this is checked for the destination.
Actually, for what you want to achieve the simpler approach is to register a IDestinationConfiguration, configure a single destination containing repository user and password. For each user you can then create an RfcCustomDestination, for which you set the logon credentials. Thus, NCo runtime is also taking care of invalidating outdated connections when the configuration of the parent destination changes.
Best regards,
Markus