Note: Flawless work and compatibility with other features are not guaranteed.
Out-of-the-box EspoCRM allows to assign only one user to a certain record. It's possible to add the ability to assign multiple users for a specific entity type.
For this you just need to create a many-to-many relationship between the needed entity type and the User entity type, with the name assignedUsers. Link Multiple Field must be checked.
Make sure that you chose link names that do not already exist in the system.
Make sure that the old Assigned User field is not set as required (at Entity Manager > fields). Make it not required if it is.