Alliance Challenges can be activated by any member of an Alliance, for members of that Alliance to participate in.
There is a lot more involved than standard Quests, in that they require a lot of materials to be gathered in order for the Challenge to unlock so that it can be attempted. Apart from the supplies that need to be gathered, Alliance Challenges function just like a Boss Challenge.
All Alliance challenges have a timer. In order to participate, you must be a member before the event starts. Failure may result in the loss of materials and no accrual of alliance points,