sp_who (and sp_who2) work great for a quick view of what's going on in the system, and can be filtered by user, but there's no way to filter by database. This quick stored procedure provides data on which users are accessing a specific database.
One of the issues you'll face with SQL Server is blocking which is caused by other processes that are holding locks on objects. Until the locks are removed on an object the next process will wait before proceeding. This is a common process that runs within SQL Server to ensure data integrity, but depending on how transactions are run this can cause some issues. Are there ways to get around blocking by using different indexes to cover the queries that may be running?