To remove a participant from a chat, perform the following steps:
Within a chat, tap the "Chat menu" button (on the upper right corner of the screen, three lines or squares).
Tap "Manage chat members." > Tap on the menu next to each member name > Tap on "Remove from chat"
Go to your Chat List > Select Chat to which you want to add people > Tap the Chat Settings Button (in the upper right corner) > Select members > Edit > Choose the contact you wish to remove > Click on the trash can icon.