I say, ask. At a minimum, make sure the person knows you are there for the reason stated by StanSeven. Also agree that you should offer to do your share.

Now for the other part of the equation - etiquette once you are in a paceline:

1) Don't accelerate when you hit the front. Please. There is nothing worse than being behind someone who thinks they have an obligation to up the pace 5 mph every time they put their nose into the wind.
2) For God's sake, learn how to ride in a straight line before you start trading wheels. I lied in #1 - squirrelly riders in a pace line are the worst. And they will get dropped without mercy.
3) Don't take 15 minute pulls up front. You will die. Keep it to no more than a minute if there are two of you, less if the other rider is clearly stronger. With four of you, count to ten and swing off. These are guidelines, not hard and fast rules - if you are teh locomotive that ride and are towing the others, adjust times in fromt accordingly - but the point of the line is for everyone to go faster by working together, not to kill yourself. If you feel fried on the front, you've been there too long.
4) When you swing off, drop back gradually, using that time to grab a quick drink if you want. Don't go for the bottle when you are in the line or at the front.
5) When you are in the line, try not to focus on the rear wheel of the person in front of you - that's a sure cause of getting mesmerized and not reactng when you need to. Try to keep an eye several riders in front or, if you can, on the road on front of the lead rider so you can anticipate what's coming and how to respond to it.
6) Don't overlap wheels. Remember, when wheels touch, the rider on the touched rear wheel may go down, but the rider with the touched front wheel will go down. Pretty much every time.
7) Above all, be predictable. That means not doing anything - turning, braking, anything - suddenly unless it is absolutely necessary. Being able to do that requires paying attention and a lot of #5.
8) If you are starting to fry and don't think you can keep up the pace, try very hard not to drop off until you reach the front and can swing off and just not latch back on the back. Sitting up or swnging off when in the middle of the group may cause an accident, and will definitely force others to have to jump the gap or lose the line. Kinda not cool, although sometimes unavoidable.

Do these things and you will be welcome in almost any pace line you feel strong enough to join.