“Everyone’s welcome” is easy to put on a poster. What happens when someone actually arrives on their own is the part worth planning.
Tell them how joining works.
If solo guests can join a team, say that in the event description. If you’ll introduce people, explain how. Specific information lets a potential guest picture themselves there. “Come alone or bring friends; we’ll help you find a team” does more work than another sentence about a friendly atmosphere.
Build an easy first interaction.
Give people a shared task: choose a team name, compare an answer, look at a clue together. It is easier to start talking when there is something to talk about. Keep the first task simple enough that nobody feels tested before they’ve settled in.
Let people participate at their own pace.
Some guests love an introduction to the whole room. Others would rather disappear under the table. Avoid putting newcomers on the spot. Offer a place, an explanation and an invitation. Notice whether they want more help or a little space.
Check the room, not just the schedule.
During a break, look for the person who still has no one to talk to, or the team that has quietly stopped following. A small introduction or clarification can change their evening. That is part of facilitating, even when it never appears on the running order.