This was a late addition to the patch notes made yesterday evening:
So that's why my 4-man guild group randomly got a 5th guild member...
I imagine a conversation like this:
Annoying guild guy> lol HEy guise! caRry me threw a roic lol
Normal guild member> Sorry, we just formed our group and queued. Too late.
Normal guild member> Maybe another time.
Annoying guild guy> lol drop grp i needz gearz lol
Normal guild member> Too late. Already queued. Maybe next time.
Normal guild member> Phew! We made it just in time!
Normal guild guy> /cheer Good job. Fast queue time FTW!
Annoying guild guy> hey guise! WOW!11! we got in teh same grp. awesome!!!1!!
Normal guild member> FML