"Who is eating in the restaurant?"
Translation:¿Quiénes están comiendo en el restaurante?
1) Quién es ella - who is she.
2) Quiénes son ellos - who are they.
3) Quién está comiendo aqui. - who is eating.
4) Quién están comiendo aqui. who is eating.
In 1 you use the singular form Quién because *ella is a singular element.
In 2 you use the plural from Quién because *ellos is a plural element.
In 3 and 4 there isn't a singular element or a plural element so you can use either forms. However,there is a small difference. When using "Quién" you are assuming one person does the action. When using "Quién" you are assuming more than one person does the action.
¿Quién está comiendo? who is eating (you are assuming one person is eating)
¿Quiénes están comiendo? who is eating ( you are assuming more than one person is eating)