How can Domain and Subdomains work on one Database?

We want to build a WebSite for our Company, which has to have Subdomains, so that each subproject can be attended by one developer. But all subprojects should work with the same base of Users, Roles, etc.
And no matter how the user came to the domain tree, he should login once and have his accountdata.
How can the subprojects read and write data of/in the main project?
 
Someone there who can help?
Thx Kirsten