Create new groovy class:

package whatever

class Person {
    def first
    def last
}
class PersonMixin {
    def getFullName() {
        "${first} ${last}"
    }
}

Person.mixin PersonMixin
 
def person = new Person(first:'Hugh',last:'Jackman')
 
assert 'Hugh' == person.first
assert 'Jackman' == person.last
assert 'Hugh Jackman' == person.^


..when using code completion on the ^ location, it doesn't show getFullName which is added to the Person class through the mixin.