"They are inviting their grandparents."
Translation:Ellos invitan a sus abuelos.
In Spanish, when the direct object is a person, it is preceded by the preposition “a.” This word has no English translation.
Jorge llama a María. Jorge calls María.
From the perspective of the English speaker, the personal “a” appears to be an extra word. From the perspective of the Spanish speaker, the personal “a” is required, and to not use it is a serious error.