I have searched for this and found no definitive answer or answers.
This website says it is a parliament, where as this one lists several others (conventicle, gulp, mischief, tidings or tittering) and an old BBC article says tiding, charm or gulp.
None of them have references and the popular online dictionaries don't seem to list a group of magpies as one of the definitions for any of these words. Compare this with "murder", which they do define as a group of crows.
This question has also been asked on Quora, but the responses seem to be verbatim quotes from the above sites, again with no references.