The use of relative pronouns quien/quienes, el que*, and el cual depends on whether or not they are preceded by a preposition. When preceded by a preposition, they can be used in both defining and non-defining clauses. However, when they are not preceded by a preposition, they can only be used in non-defining clauses. For example:
Non-defining:
La chica, quien estuvo en la reunión, es fotógrafa.
The girl, who was in the gathering, is a photographer.
La chica, con quien fui a la reunión, es fotógrafa.
The girl, with whom I went to the gathering, is a photographer.
Defining:
Esta es la chica con quien puedes hablar de arte contemporáneo.
This is the girl with whom you can talk about contemporary art.
Esta es la chica quien estuvo en la reunión.
*Note that el que is often used after the verb ser ("to be"). In this case, the sentence does not need to be delimited by commas:
Esta chica es la que estudió conmigo.
This is the girl who studied with me.