The first dimension is group, the second is group members.

There's some scary terminology here, but it's not at hard as it sounds. In different terms, musical_groups is just a list that contains a bunch of other lists. All those lists contain strings, which you need to join with a comma (,).

You will need to write a for loop here to get each list in musical_groups, but then you can join each one with the join function on str (which it looks like you've caught on to, though you'll need to call it on a literal string "," instead of on the currently-undefined variable comma).

For example, this does a similar thing, but adds together a bunch of numbers:

matrix = [
        [1, 2, 3, 4],
        [5, 6, 7, 8, 9],
        [3, 2, 8, 5]
]

for row in matrix: # This grabs each list in the matrix array, one at a time, and assigns them to row
    print(sum(row)) # This will add the numbers in each row together and prints them out

The above code will print something like this:

>>> 10
>>> 35
>>> 18

print(", ".join(group))

musical_groups = [
    ["Ad Rock", "MCA", "Mike D."],
    ["John Lennon", "Paul McCartney", "Ringo Starr", "George Harrison"],
    ["Salt", "Peppa", "Spinderella"],
    ["Rivers Cuomo", "Patrick Wilson", "Brian Bell", "Scott Shriner"],
    ["Chuck D.", "Flavor Flav", "Professor Griff", "Khari Winn", "DJ Lord"],
    ["Axl Rose", "Slash", "Duff McKagan", "Steven Adler"],
    ["Run", "DMC", "Jam Master Jay"],
]
# Your code here
for row in musical_groups:
    print(", ".join(group))

My code isn't working. Can anyone help?

Nevermind I figured it out.

for group in musical_groups:
    group = ", ".join(group)
    print(group)