Hi,
Can you provide a bit more detail on the purpose of the app you're building and why users need to sign-up or in?
The paid version of RStudio server had this feature built-in I think, but you could build something similar for free with the shinyauthr package for example.
The security of these approaches is probably not as good as industry standards, so I wouldn't use it for highly sensitive data...
Hope this helps,
PJ