For K2 to allow users to start a process I find its typically best to add an AD group that contains the users that have permissions to start the process.  If its everyone in the company then I'll normally add "Domain Users" and provide them with Start permissions. 

Note that you don't have to specifically give users permissions to perform tasks inside a process.  When a user is assigned a task as a destination user for an activity that user is automatically given permissions to perform the task. 

There are actually a lot of options about how you can assign the tasks to inside a process (person's manager from AD or looking up a list of users from a SharePoint list).  You might want to give the following whitepaper a look for more details about desintation user options:  http://help.k2.com/en/AdvancedDestinations_Whitepaper.aspx

Thanks for the info. That helps. Just to clarify, after you publish each process you still manually go in and add permissions? There is no default that you can set to add users/groups when a new process is published?

No there is no default permissions set except that the publisher (exporter/deployer of the process) automatically gets Admin permissions (which is actually a relatively new feature to K2 as in the past even the person deploying it wouldn’t be given start permissions automatically). I've never found this to be an issue since the processes I've worked on typically require different start permissions anyways.

However, once permissions are set for a particular process they are not reset when you deploy a new version of the process which is good as its normal to spend a lot more time deploying new versions of a process then complexly new processes.

Thanks Tim. I appreciate your help