Title says it all. Is there proper etiquette for introducing new, eager players to an existing campaign? What are your personal methods. I try to have it be story related for the most part. Intersecting new adventure hooks that are both part of and deviating from the main objective. Do they get all the levels Pc's have earned for free? Or be lower lever and risk being an annoyance more than a welcome edition?

Thanks again. Happy DMing, all.

I tend to prefer to keep all PCs the same level.  I also try to work new PCs into the story, or at least come up with an IC reason for their existence, not just randomly change the composition of the party and act as though it'd been that way the whole time.  If one of the world's gods takes it into Its head to do that, though, it's not my fault. 

I have characters pop in and out periodically.  I try to tailor it story flavor.  My only "that can't happen right this second" rule is that if a party is deep in some dungeon, the new comer just hang out for a bit until the surface.  Luckily, my players believe the same thing.

I've had a couple of new players (and new characters for old players) enter the game. The party was back in town for one session, so 1 just joined there. Another was a prisoner they rescued. One was a bad guy who switched sides in a battle. A bit awkward but it got the job done.

If a new player is joining, I always try to weave them into the story.  Whatever the party's current goal, give the new player the same goal, but from a different angle.  I'll usually wait until there's a point in the story where it will take several sessions to accomplish the goal; if it's too quick, the bond formed might be too tenuous to stick together for the next quest.

As for levels, I like to put them at the same level, but at the minimum xp amount for that level.  So there will be some party disparity, but not a great deal.

I run an AL style game where new players are anticipated (although in practice there is a group of regulars).  In terms of incorporating them, several stock answers (prisoners, encounter in town, etc.) have been suggested.  In the wilderness, another option is to encounter another adventuring party or the remnants there of.  In terms of level, I have new players start at the lowest level of our regulars.  (For example, currently our group is a mix of 4 & 5 level characters)  But I also keep the level difference between regular players PCs to one level.  E.G.  the first PC to reach level 5 did so, any level 3 PCs leveled up to level 4.  This rewards players for being more regular, but doesn't penalize those who aren't too much.

In our current campaign (TOA), when the party composition between regulars shifts, I do deus ex machina the changes:  a few adventures back, the party befriended a fey dog named Pondi.  Pondi now periodically "kidnaps" party members to play with them and then eventually returns them.

My players are usually lost and in a mess so I introduce the new players also as lost and hopefully their lifeline.