Raid question - helping other guilds.

[Morgander]

Simple question but no matter who I ask (including petitioning), nobody can answer the question.

If someone from a class C guild wanted to help a class R guild with a raid mob, can they?

If so, what's the limit? 1 person? 2 people?

And if not, what prevents someone from just swapping guild tags?

What if you have a character in both guilds?