Adding extra field to a specific form instance in Django formset

Question

I'm making a registration form for an event using a Django formset. All of the forms in the formset have 2 fields, mem1 and mem2, except the first one. How do I add an extra mem3 field to the first form?

Here's my forms.py:

class EventTeamForm(forms.Form):

    event = forms.BooleanField()
    mem1 = forms.CharField(label="Member 1", max_length = 20, required = True)
    mem2 = forms.CharField(label="Member 2", max_length = 20, required = False)

class BaseEventTeamFormSet(BaseFormSet):

    def clean(self):
        if any(self.errors):
            return
        for form in self.forms:
            participating = form.cleaned_data.get('event')
            mem1 = form.cleaned_data.get('mem1')
            if participating == True:
                if mem1 == '':
                    raise forms.ValidationError("The 'Member 1' field is required.")
            if participating == False:
                if mem1 != '':
                    raise forms.ValidationError("Please tick the required event.")      

    def add_fields(self, form, index):
        super(BaseEventTeamFormSet, self).add_fields(form, index)
        form.fields["Member 3"] = forms.CharField()