I am climbing alone. Can I join a group on Kilimanjaro or another of the 7 summits?

I am climbing alone. Can I join a group, and how do I find one that suits me?

Yes. Most operators run open group departures on fixed dates that solo climbers join, and on Kilimanjaro, Aconcagua, Elbrus and Everest that is how most people climb. The operator directory shows who runs which peak.

The full answer, kept up to date: I am a single climber. Can I join a group?