Kenkyuusei literally translates as "research student". These girls are basically in training. They substitute for regular members who are absent from stage shows, among other activities. Eventually they are promoted to a regular team, dismissed or graduate from NMB48.
It was announced that Team M will be created from the current kenkyuusei in late January. The NMB kenkyuusei are currently performing a revival of Team A's first stage, "Party ga Hajimaru yo".