Rules may differ depending on the host that sponsors, schedules, and plan the event, but as a general guideline:

For Sign-Up Based Events:
For fairness' sake of the people that gave prior notice of interest in an event:
1. First priority for spots is given to those that have signed up and specified the preferred roles.
2. Second priority is given to those that have signed up but haven't specified a role or profession.
3. Third priority is given to those that are on reserve(those that are interested but not necessarily capable of being there, etc)

Anyone not signed up in any form may be filled in for remaining spots, but that is a case-by-case situation.

For All Events:
In most cases, events are announced and posted ahead of time, out of respect and consideration for the people that may need to change their plans or to make sacrifices, spend their time and effort to make the event possible. Therefore:
1. Cap elites and learn skills ahead of time, especially if specific builds and recommended skills are mentioned.
2. Get your preferred characters to the needed outposts ahead of time.
3. If you think there are alternatives that could be more effective, mention it to the host ahead of time, be it better time/date, better builds, etc.
4. Even if you don't have a particular profession, build, skill set, etc, ASK. It is up to each host/sponsor of the event, but exceptions can be made as long as you ask ahead of time.