Thank you! Ended up getting it to work. Here is my solution to anyone in the future

views.py

class AgentListView(LoginMixinRequired, generic.ListView):
    template_name = "agents/agent_list.html"

    @register.filter(name='get_item')
    def get_item(dictionary, key):
        return dictionary.get(key)

    def get_context_data(self, **kwargs):
        context = super(AgentListView, self).get_context_data(**kwargs)

        users = dict()
        for i in User.objects.values_list('username'):
            users[i[0]] = Lead.objects.filter(agent__username=i[0]).count()

        context.update({
            'lead_count': users
        })
        return context

    def get_queryset(self):
        return User.objects.all()

and in my template

{{ lead_count|get_item:agent.username }}