Well, after a bunch of research, I ended up using a value converter. I made it OneWay so it only converts, but doesn't convert back - here's the code I used
<CheckBox.IsChecked>
<MultiBinding Converter="{StaticResource CollectionToBoolConverter}" Mode="OneWay">
<Binding Path="ID" />
<Binding ElementName="Me" Path="User.GroupList" />
</MultiBinding>
</CheckBox.IsChecked>
Where ElementName=Me is specified up top as the form I'm using.
Hopefully this helps someone else out.