When initiating a task, that should be assigned to a single user. After release that task should visible to all the users in a given userlist
No, it's not directly possible.
The custom filter will calculate a list of users to be able to see / "take" the task. The custom filter does not assign a task to a specific user (except one use case: if the list of users is only one user, then it is automatically assigned to this user).
So, what you have to do is:
1/ first, calculate the list of user, by the actor in the lane. Then, all users will see the task, and nobody is assigned.
2/ on a second mechanism, automatically assign the task to one user.
How to do that?
* by the Event Handler mechanism. You can execute your JAVA class when the task is "READY"
* by an external tool, which searches tasks and assigns them automatically
* Create a process with a timer, executed every 10 mn, and in Groovy, search tasks and assign them automatically
ATTENTION is it not recommended to use a process for technical usage: you generated a lot of cases in the engine. One case every 10 mn => 6*24 cases a day, 6*24*365 = 52560 cases a year
* add a plugin in the Truckmilk scheduler to fit this need