Sometimes you can not make the raid from the beginning. We all respect that real-life is a priority.
Although sometimes it is not possible to join a raid in progress because of the mechanics of the particular encounter or zone, most often you can join the raid later - either at any time or during certain stages. If you do, you should follow certain guidelines to make it as easy as possible both for yourself and the raid.
In any case, getting to the raid when it's in progress will take some effort on your side. Although the raid leader and the other people who are already with the raid will help you as much as possible, their first duty is with the raid. The biggest part of responsibility of getting to the raid will lie on the person who wants to join the raid in progress.
First of all, you will need to travel to the zone where the raid is taking place. We are repositioning the guild banner as we move, so most often it's possible to take the guild portal directly to the raid, but even that is sometimes not possible. If you know in advance that you will be late and if the raid is planned in advance as well, the best thing to do is to run to the raid zone some time before the raid and camp there (ideally fully buffed); that way when you log on during the raid, travelling will not be an issue.
You should also join the raid channel as soon as you log on and ask about the general situation (especially where the raid currently is and what you will need to do to join the raid). Sometimes this information is in the guild message of the day, so please check it before asking. If noone answers your question in the chat channel, ask again; don't send a tell to the raid leader.
Join the class-specific channels as early as possible as well; doing so will allow you to ask about your specific role in the raid early, so you will be able to start playing an active role at the moment you get to the raid.
You should then follow instructions that you are given. Sometimes joining will be as easy as getting an invite to the raid and being CoHed, but sometimes it will be more difficult: you may need to run to a specific place in a zone, you may need to die and be dragged to the raid, you may need to wait a significant amount of time until the raid gets to a spot where it is possible for you to get, or something completely different.
Sometimes getting you to the raid takes time, so please be patient. The people who are with the raid may be fighting, the raid leader may be busy explaining strategy, the CoH mage may be buffing the raid, and so on. They will not ignore you; they will get you to the raid as soon as possible. There will even be raids where join-in-progress is not possible at all, mostly in instanced zones where the task is locked soon after starting or zones where the guild banner placement is disabled.
When you successfully join the raiders, you should ask for buffs that you need. See the Buffing section for information about how that should be done. Do not forget to shrink as well.
You will also need to ask about tactics and role delegation; the best place to start is the raid manager window, where you can get information about chat channels and roles of each person (on the notes tab), and you will also be able to see who the Main Assist is (that person is marked by an A in the group/member listing). Ask in group or in the raid chat channel about things that you can not find in the raid manager. Remember this is your responsibility to ask when you join in progress.