To add functionality just after the user login, you have to modify the user-exit SUSR00001 (transaction CMOD)....
By submitting your email address, you agree to receive emails regarding relevant topic offers from TechTarget and its partners. You can withdraw your consent at any time. Contact TechTarget at 275 Grove Street, Newton, MA.
Here you can add message display, ...)
You can also modify the Start menu (S000 by default) for each user:
Go in to transaction SU01, constants, menu.
The menus can be modified with transaction SE43.