Answer
Open the chat name or More control to show Chat Options, choose Chat Participants, open More beside that person, then choose Remove Participant and confirm. Add People handles invitations. There is no participant-removal swipe gesture. If you are not the admin, ask the current admin to act.
On updated builds, repeating an already completed removal is silent. Adding a person who is already present does not add another activity or notify them again. A later deliberate re-add remains possible through the normal controls.