This is a post for GMs of games. GM is a term from tabletop roleplaying, in OngoingWorlds we normally use the term “owner”, or whomever created the game to start with. Anyway, if you created the game – you’re the GM – the one responsable for everyone else to enjoy it!
Plots can be varied and wide in scope, but it’ll normally be about a bunch of characters doing stuff. If you’re doing it properly, everyone’s character will be involved, with nobody left out.
If there is someone who’s being left out, make sure you involve them. You can involve them by making sure there’s something for them to do. Maybe they’ve got a unique skill that needs to be used, for example where’s a door that needs to be opened, and the character who’s being left out is the only one who can lockpick it. Or maybe s/he’s the only one strong enough to open it.
What you don’t want is a situation where one character feels like they’re being constantly left out because they don’t bring anything to the table. This could be that they’re surrounded by people who are more powerful than they are (see the article about Angel Summoner & BMX Bandit), in which case they need to make their skills more useful.
Or you as the GM need to steer the plot so that everyone’s unique skills are useful.