In-Process API GroupUserMembers.Create GET Works, POST Fails

I'm putting together a Plugin handle some data and then add a user to a group programmatically. I'm using a Widget to call the method on a page. When I send the data via querystring everything works fine. When I send it via POST everything works except for the call to Telligent.Evolution.Extensibility.Api.Version1.PublicApi.GroupUserMembers.Create(). Both methods pass in identical data to the Create method, but only when using GET does it actually add the user to the group. Using POST it returns a null GroupUser.

Is this expected behavior? Should you not be able to post to a Page and use InProcess calls? If that's the case, what is a better way to go about doing this?