Create a new public channel for planning the birthday party, and add the planning committee as well as the cross-functional friends.

Create a group direct message (group DM) with the two members of your team since they are the only ones involved in the planning process.

Create a thread for planning in your existing team channel and @mention the team members on the planning committee.

Create a new private channel for planning the birthday party, and add the planning committee as well as the cross-functional friends.