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?