It's a bit tricky to assign task to a variable number of users. The Gateway service is good, but when you know the number of user ahead of time.
If you don't know the number of users ahead of time, you'll need to be a bit creative and use the Gateway service (to assign the task to a user) and then loopback to it n number of times so that multiple users will get the item.
Then the other part you'll need to worry about is when users will complete their task. You'll need to have a condition outside of the Gateway to check if all users a done, before moving forward with the rest of the process.
Jasmin