Board Thread:The Bulletin/@comment-195.220.159.34-20160307143341/@comment-25085701-20160308204200

Definitely a guild. In a Party, you would:

1) have to individually invite all 100 people  (public guilds can be joined by anyone who has the name of the guild

2) need to deal with having 100 people's possible damage on any given day if they miss dailies (much more likely chances of taking enough collective damage to kill people)

3) possibly wind up with slower load times (since when you log in, it calculates the damage to you and your party for any missed dailies as it processes your cron)

Challenges work the same whether they are in a Party or Guild, so for a challenge of that size, definitely go for the Guild.