Create CSV file of domain groups and their users
Some notes:


Comments: A lot (if not all) of the comments are pretty much repeating what the code already says. I'd remove them.
server = # enter server name here. Create functions with configurable elements as arguments instead of global variables.
with open('groups_members_output.txt', 'wb') as blank_file. Try to give more meaningful names to variables, be declarative. blank_file says hardly anything useful about the variable.
grps/usrs: Just for saving one or two characters now variables are harder to read.
Put spaces after commas.
Use (functional) list-comprehensions expressions instead of (imperative) for-loop statements whenever possible.
Use always "\n" for line endings (read and write), Python takes care of portability in all platforms.


I'd write:

import win32net

def write_group_members(server, input_groups, output_groups):
    with open(input_groups) as input_groups_fd:
        groups = [line.rstrip() for line in input_groups_fd]

    with open(output_groups, "w") as output_groups_fd:
        for group in groups:
            users = win32net.NetGroupGetUsers(server, group, 0)[0]
            for user in users:
                contents = "{0},{1}\n".format(group, user["name"])
                output_groups_fd.write(contents)

write_group_members("someserver.org", "groups.txt", "groups_members_output.txt")