A group member from my last game made a few menu bottons i wanted to use in this game. Would that be allowed or do i have to recreate them from scratch.

By context I'm assuming the person who created the buttons is not on your current team (if they are, it's "theirs" and you can just use them)

You can use those *AS LONG AS* you cite source (Button art by John Doe) *AND* you have their permission for use.  TOTAL sourced assets (from all sources) cannot exceed 10% of the project.