A new user
New users are created in the admin console: name, email and role. The system role has three levels — system admin, admin, regular user. Workspace and project permissions are granted separately.
When someone leaves
Rather than deleting the account, transfer their open tasks first — there is a dedicated action for it: every open item moves to the colleague you choose. Nothing is lost and the history stays.
Merging duplicate accounts
If the same person ended up with two accounts (different email addresses, say), the accounts can be merged — tasks, comments and history gather under one.
A merge cannot be undone — settle which account survives before you run it.