they dont do either its usually just word of mouth. friends of other guildmates join because they like what they hear about the group. the only vetting they do is have enough gear to not get one shot by mechanics.
we grow and shrink of course but twenty is pretty average for the majority of the tier. it will usually be full to the point of splitting the raid at the start of the tier but there is a pretty core group of raiders that have been around for a long time in the guild. as far as the guild leader has told me he joined the guild in wrath and it has moved servers a couple of times and factions but they have been around a long time.