Before we can kill the first monster, we need to set up the raid, buff, and move to the raid zone. All these things take some time. If we started to prepare the raid at the scheduled start time, we would not kill the first monster until much later. Ideally, everyone should be on the gathering spot between 15 and 30 minutes before the raid is scheduled to start; that leaves enough time to set up the raid and groups, buff, and explain the basic information about the upcoming encounters.
It's everyone's own responsibility to get to the raid rally point. While ports may be available and while we will usually have the guild banner set at the rally point, there are situations when this will not be possible - so please know how to get to the raid zones, especially the hub zones that we're mostly gathering in.
The raid leader usually invites several people to the raid, who will in turn invite other people to their groups, and when everyone is invited to the raid, the leader can then make some changes to the group setup. When group leaders handle most of the invites, the raid leader can organize other things (like delegating tasks to others) at that time. To make this process as smooth as possible, you should ask for invites to the raid in the raid chat channel (usually dofraid). Before doing so, please disband from your group before asking - it will make life much easier for the group leaders.
If someone is not flagged for a particular zone, it's possible that they may still be able to join, as long the raid has enough raid-in spots. But there is no guarantee, and there is also no guarantee that even if you get a raid-in spot in the end that you will be able to stay to the end. If the raid-in quota decreases, perhaps because a flagged person goes linkdead or has to leave the raid, some of the unflagged people may be ported out of the zone. If that happens, there's nothing we can do about it - tough luck. The required flags for the content we're raiding are known well in advance and the tasks needed to get the flags are usually easy (this is especially true for a Tipt trial in Gates of Discord), so there's absolutely no reason not to have the flags.
Some zones, like Qvic, have backflag quests that require a drop from that zone and then some group content. This backflagging option makes it possible for a guild not to repeat the flagging raids over and over again, once the majority of the guild is flagged, so please take advantage of the backflagging. Yes, it will need some work and it will need some help from a few friends, but it's much better than asking 50 people to re-do the raids several times.