Retrieving all chat members

In order to get all the members from a mega-group or channel, you need to use GetParticipantsRequest. As we can see it needs an InputChannel, (passing the mega-group or channel you're going to use will work), and a mandatory ChannelParticipantsFilter. The closest thing to "no filter" is to simply use ChannelParticipantsSearch with an empty 'q' string.

If we want to get all the members, we need to use a moving offset and a fixed limit:

from telethon.tl.functions.channels import GetParticipantsRequest
from telethon.tl.types import ChannelParticipantsSearch
from time import sleep

offset = 0
limit = 100
all_participants = []

while True:
    participants = client.invoke(GetParticipantsRequest(
        channel, ChannelParticipantsSearch(''), offset, limit, hash=0
    ))
    if not participants.users:
        break
    all_participants.extend(participants.users)
    offset += len(participants.users)
    # sleep(1)  # This line seems to be optional, no guarantees!

Note that GetParticipantsRequest returns ChannelParticipants, which may have more information you need (like the role of the participants, total count of members, etc.)

The hash of the request is not the channel hash. It's a special hash calculated based on the participants you already know about, so Telegram can avoid resending the whole thing. You can just leave it to 0.