use a kanban board. FOSS options exist for web-hosted implementations. Kanban works because It supports a 'backlog' of events to do, timelines for things to do that are on a deadline (e.g. concerts) and allows people to add activities / plans to backlogs which get moved to 'ready to do' as things are done / cleared. Keeps everything organised and simple, with good over-sight on what's going on when. Downside is it requires friends already somewhat technical or at least willing to learn.