Then approach is
- Get reference to current clientcontext
- get web site reference
- get reference to existing list
- break the inheritance
- define a role
- get the user
- finally add user and related role to the list
How to Add EveryOne (All Authenticated users) to permission group
in here you need to get the user which represent all users. Therefore you can use following code to archive that.
these are the role definitions you can use.