Girls girls girls, calm dowm. Last time the superbowl was in Miami they also needed some people on the field for halftime. The super bowl is given to a city to organize, it is more the city's celebration than it is the fans'. In any case what I mean to say is that just like the state at which it is held is given a bigger percentage of tickets to distribute so are the locals given more opportunity to participate.
Usually there's no need to look for volunteers as enough people call the stadium asking to volunteer. If they need a specific type of person, like kids, they simply get the public schools involved. Lately I think they've used agency type things (who only work with non-paying gigs) to get the word out (NOT recruit, they simply contact everyone in their list looking for volunteers), mainly cause an agency keeps track of people so it's better for both security and just reliability reasons. If all else fails and they still need volunteers they get radio stations involved, contests, etc.
So while U2 could have just asked for fans to join, they would be responsible for the major paperwork and duties involved. Meaning they'd have to set up camp here and hire people to take care of everything all for an event that will last at most 15 mins. Why cause such chaos when there are people already in place to do this? Not to mention it would be majorly disrespectful for the citizens of the city who won the bid for the superbowl as to be part of it by volunteering is supposed to be one of the perks.
So now breathe in, breathe out, repeat as necessary.
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Long live the lemon!!!