multi user increment

Hi All

Any thoughts on how to address this problem

I have a multi instance app that connects to a single data store.

For any user, across the instances, I need to increment a numerical  
value newly for each new registered user.

This then has to be saved back to the data store and for there to be  
no conflicts with similar values for new registrants...

Any ideas or have I missed something easy here..

Gino