Most of the time yes, HPC systems are shared among many users. Sometimes though the whole system (or near it) will be used in a single run. These are sometimes referred to as "hero runs" and while they're more common for benchmarking and burn-ins there are some tightly-coupled workloads that perform well in that style of execution. It really depends on a number of factors like the workloads being run, the number of users, and what the primary business purpose of the HPC resource is. Sites that have to run both types of jobs will typically allow any user to schedule jobs most of the time but then pre-reserve blocks of time for hero runs to take place where other user jobs are held until the primary scheduled run is over.