This part can be a bit awkward because it runs into SynthV’s “grey on grey on grey” issue.
You can “enter” a group by either double-clicking one of its notes, or by selecting the group marker at the lower part of the piano roll (below the first note of the group). You’ll know you’ve “entered” the group if the sections of the piano roll outside it to the left and right are darkened (this is the part that might be less visible on low-contrast displays).
At that point you’ll be able to select individual notes and adjust things as usual for the notes within the group.
You can then “exit” the group to return to the main track by clicking the indicator at the bottom again.
There’s also a “Go to Group” option if you right click unoccupied space in the piano roll, which can be helpful if you get “lost”. Personally I have a bad habit of scrolling away from a group without exiting it first, and this is an easy way to return to the main track regardless of what’s actually visible at the time.