You’ve been running your campaign for a while and your collection of players has dwindled from the desired six to a barely tenable three. You’ve made a few semi-permanent NPCs to help lighten the load on your remaining players and you’ve changed all the encounters so that your primarily martial characters can still fight on an even playing field. Still, you and your players feel the lack of other voices around the table, other solutions to the problems you face that could be offered by one or more other players. Maybe you have some interested people who’d be willing to play the kind of game you’re running, but how do you know if they’ll fit into the group dynamic? How do you know if they’ll really enjoy the story you’re all telling when they’re not as invested as your current players. Assuming you get past the first two, how do you work them into the campaign without it feeling like you’ve put everything on pause so a new character can show up in order to bail out the party?
Adding new players into an existing campaign is always a risky proposition. There is no telling what a new face will do to the group’s chemistry or how the leadership or problem-solving dynamics will shift as you add new personalities. A lot of the potential problems can be avoided if you bring in a prospective new player on a temporary basis, for some kind of special event cooked up for the sole purpose of vetting new players. Keep in mind, no matter how well you know the prospective player, it is really important to give the other players a chance to try them out first before you bring them in officially. There is always the chance that a quirk of someone’s personality will be incredibly frustrating to someone else, even if they usually get along or you don’t see it. Since your existing players have been with you all this time, they should ultimately have a say in new players as well and group chemistry is just as important to them as it is to you, even if it is ultimately your job alone to monitor and/or police it.
While you may want to bring in a new player right away, to help the players out of a problem they’re approaching, it is usually best to save inserting the new player until there’s room in the story for it. Thankfully, stories are quire versatile and the reasons behind why a stranger might join up with the existing characters are manifold. Maybe the new character is a prisoner or a turncoat. Maybe they have goals similar to those of the party and found their way to the same place. Maybe the new character has some important information the party needs so they seem them out in town. Maybe the person giving the party their job wants to send someone they trust along to report back and ensure their goods are properly retrieve or delivered. There are a thousand ways to add someone to the game, but it’s just as important to know that every moment isn’t the right time. If you characters have been chasing a bad guy for months, one who has wronged them and only them, it would not make sense for a stranger to show up at the bad guy’s base with the thought of helping to take down someone who hasn’t done anything to them. Similarly, if your players are carrying out a top-secret mission, it is unlikely that they will willingly share information with a new person unless they explicitly know they can trust this stranger.
Usually, to get around those difficult moments and to help both get the character involve and make sure they’re a good fit with the group, find a little side adventure you can use that will involve the new player. You can watch the group chemistry to make sure everyone gets along and help the characters build a rapport so that your existing players will readily welcome the inclusion of any new players. If you’ve got the time, it never hurts to vet a bunch of players ahead of time, to see how they perform, in case you ever need to add some more people. I like to invite people I know to small parts of campaigns I run so I can get an idea of how they play and who they play well with so I can make sure to invite the right people to the right Dungeons and Dragons groups when I’m looking to start a new campaign. This means I usually have a good idea of who will fit well in a group if they initially declined or weren’t available and I wind up needing more players.
From there, if I realize I’m running short on players and will probably start wanting new ones soon, I go through my mental list of players and invite potential new players to join the campaign for a short little story, usually something heavily related to the main plot of the game with an individual twist focused around the player’s character. If they enjoy the piece of the story they got to experience, then it’s usually a safe bet that they will enjoy playing in the campaign as a whole. It isn’t a sure-fire method, of course. There are no sure-fire ways to predict the future or make certain that everyone will get along in the future, but it makes it a lot easier to confidently suggest people to your existing players and, if there are no red flags, then most game masters can handle it from there since any issues will fall within the normal range of personality conflicts most GMs handle on a monthly basis.
As always, you should consider things thoroughly before acting. There’s no rush to add players, so take the time to make sure you’re adding people who are going to have fun and actually contribute to a positive play environment. It might take a lot of work sometimes, but it’s always worth it.