Just in case anyone is using this course to learn English, I'll go ahead and mention this:
Because "often" implies repeated action, the best translations are:
You sing together often.
You often sing together.
Certainly a choir director could refer to a specific occasion within a work of music and say to the altos and tenors:
You are singing together often.
You are often singing together.
But I'm pretty sure that the majority of native speakers would reject as ungrammatical in any context:
*You are singing often together.
If I were grading an exam, and a student produced this last example, I would correct it to one of the first four possibilities.