Create a private or team channel and add its first members. Use private for an invite-only channel and team for a channel owned by the current user’s team. Do not use this for a direct message — those are created separately. Team id is resolved from the current user; do not invent one. Participants accept macro|<email> ids from ListTeamMembers or bare emails. Creating a team channel when the user has no team fails; create a private channel instead. Creating a team channel with no participants adds the current user so the channel is valid. Use only when the user asks to create a channel.