Streamline main script.

[ben-e-whitney]

Notes:

• The newlines in the description are ignored. See the documentation. For the same reason I have edited the help parameters for the arguments. You can use formatters if you want to put in the newlines manually.
• Since you aren't in any way handling any exception that may be raised when appending to members and names, I think it's better to just let the exception raise. Then the user will at least see what the type of the exception is.
• String concatenation was working fine, but I believe it's considered better practice to use os.path.join.
• Rather than iterating over range(len(replace[uid])) (AKA range(len(members))), we can directly iterate over zip(members, names).
• By using infile.seek(0), we can avoid reopening the file for every contributor.
• If a member hasn't given their name, the corresponding entry of names will be None and the line.replace call will fail. Using DEFAULT_NAME as a default avoids this.
• Providing the exit message to sys.exit causes a nonzero exit code to be returned to the shell. See the documentation. We can instead just print the message and exit normally.

Let me know if anything else is unclear.