The lower left quadrant comes into play when two 'I's meet and there is shared perception, connection or any emotional state generated by social interaction. 'We' then relates to anything where we are in relationship with others - communities, families, romantic relationships, workplaces, neighbourhoods, national sentiment and so on. The feelings generated by this 'we' domain can be strong yet they often go unstudied and unacknowledged.
This quadrant also relates to culture and shared beliefs as these have the power to shape perceptions not just of me as an individual but for me as a member of a 'we' group.
In terms of academic subjects/schools of thought related to this quadrant this ranges from Jung with his theory of the 'collective unconscious' to structuralists such as Claude Levi-Strauss and Roland Barthes who were interested in signs, myths and linguistic structures.
Social psychologist too, particularly social identity theorists, particularly focus on this intersubjective realm arguing often that it is as 'we's that human beings need to be understood not simply as 'I's.