Let me just confirm the question and the situation:
You used survey flow to assign people to a "Group" (by creating an embedded data field to capture this) but when you look at the results, you aren't able to use this value in reports, correct? I would assume then that at the bottom of the data somewhere, you have a field for the Group, but they are just text values?
If that is correct, then the issue is the data type of "Group" -- go back to your survey flow, and everywhere "Group" embedded data is assigned, make sure that you change the Options to make it type = Text Set. This tells Qualtrics these are not just text values, but a succinct set of values you want to use for other purposes, like filtering, breakouts and drilldown.
Once you make that change, the data will update in the back end and then Group should be available for reporting purposes.

Im not sure if I understood correctly. I can't find the option where I can make a change in the survey flow.
It would be very nice if you could explain this part again " go back to your survey flow, and everywhere "Group" embedded data is assigned, make sure that you change the Options to make it type = Text Set. " Thank you

I can explain it, but first, let's make sure I am understanding your need and what is happening: How did you assign people to Groups?
I am envisioning that somewhere in your survey flow, either statically from the contact file, or dynamically during the survey, when you determined which group they were part of, that you actually assigned that value, for example:

If that is so, then clicking on OPTIONS as circled in the image above allows you to change the data type to Text Set, which is usable in reports.

However if you assigned them to Groups in some other fashion not included in the Survey Flow, then it would be helpful if you could explain that so we can better guide you.