As you've probably figured out, there's a max of 8 characters allowed. You have to disband any member of your party (you can even dump your original hero if you'd prefer). See that interface at the bottom right of the screen?

You need to select the character you want to get rid of, then click the disband button. It's the one on top that looks like three dots with an arrow pointing to a fourth. Your selected character is gone, but if you make a mistake talk to him again and he'll rejoin.

Otherwise you can talk to Merik and have him join. Bear in mind if you'd like to switch back, you'll have to refind where you left that character.