How do I bring new players up to speed when joining mid-adventure?

Question

My group (3 PCs) and I (DM) started the Lost mines of Phandelver adventure 2 weeks ago (it's the 1st time playing D&D for all of us). We played once a week since we had kind of mixed schedules but we managed to finish one chapter per week. After we finished the 2nd session one of the players told me that for personal reasons he won't be able to keep playing with us. So we searched for other players and found two — one has played a bit before and the other is completely new to roleplaying games.

I have some ideas on how to introduce the new characters to the party but I'm concerned they'll feel left out since they've missed the previous chapter. How can I introduce the things that have already transpired during the adventure to the new players and how to not make them feel left out?

Answer 1

Let the players do the work, if possible, not you.

Honestly, two sessions is not a lot of lost time to make up for; it is below the threshold where I would actually worry about it, too much. My preferred approach would be to work their backgrounds and motivations into the overall game at large, and just let the PCs organize their own "fill in the details" as a role-playing exercise.

It's also possible to just flat out infodump the events of the two sessions to the players without worrying about how the players know things (because players often know and firewall information that their characters shouldn't have) and then ask them to come up with characters who have a reason to be in the right place, at the right time, going in the right direction to join the existing/remaining group. (With your blessing as the GM.) Just warn the existing/remaining players that you expect them to accept the offers and justifications.

A comment below makes a very good point which complements my initial answer. In addition to satisfying my basic laziness, letting the players do the heavy lifting can also inform the GM of a number of things, including:

1. What the PCs understand about events
2. What the PCs think they understand about events
3. What the PCs are interested in
4. What the PCs are planning, if anything
5. Etc

Even if your players are up front about this with you, the GM, it is quite possible they will say something different to the new players.

Answer 2

When introducing new characters you have a couple of options right off the bat.

1. Handwave it: Although certainly not the most immersive method, depending on how consistent your players are you can just have them be in the party no questions asked. This is fairly effective in published adventures that are not directly related to indavidual player backgrounds.
2. "Wow same quest!": A small amount better in terms of immersion you could have the new characters be found by the current party doing the same thing. Are they attacking a goblin cave? well so are the new characters! what are the chances?
3. Custom Tie In: This one is the hardest but also best in terms of adding new permanent members whose backstories you want to tie in. Work with the players background (and the player) too see if you can find a reason they would be in the same location as the party and would want to join the adventureres. For example: in Lost Mines, maybe one of the new characters is in Phandalin and is sick of being bullied by the Redbrands. Perhaps they are searching out the Hag for their own reasons. Are they treasure seekers themselves? the lost mines are rumored to have a secret treasure that they could be looking for.

Answer 3

The best answer for this is a variant of the first one - "Make the players do the work for you" but revolves around two simple questions:

How do you already know these guys? AND Why do you trust them?

If the players answer these questions, you are all set. Let them make up whatever they want as long as it makes some modicum of sense - "This wizard helped my mercenary company with identifying some magic runes!" or "This cleric is my old drinking buddy!" or whatever.

Past history. Characters don't magically spring into existence at level 1, after all.