I'm running a West Marches style game and I'd like for players to have a shared map they can edit. What's a good way to handle this in a way that can be shared with all the potential players between sessions?
I'm also running a West Marches style campaign for a large (25+) group and we've found that Google Slides works really well for this. You can create a map using some other cartography software and set it as the background for the slide (or break it up into multiple slides to zoom in better) and then add sites / routes as textboxes on top. You can either allow the players to edit the slides directly (they can add more text to overlay the map, or they can add more slides of notes which they can link to from the map) or you can give them 'Comment Only' access if you're worried about them moving / deleting things.
The map is accessible really easily (via a url) and can be viewed / edited by multiple people at once no problem.
A friend of mine made a tool after wondering the same question. We now use on a regular basis: Hextml site here and it is free.
Players manage the player map
Give responsibility for the map to the players. Let them figure out a way of sharing it. Offer to send/share a link or a file if they provide you with one.
If the players do not share the map, then new players start without and should probably produce their own. The situation should be fixed without a couple of sessions, as recurrent players are likely to have access to a map through some means.
A Large Sheet of Paper
I've been running a West Marches campaign for some years now. I have tried various online solutions and not found any of them nearly so satisfactory as a sheet of A1 paper folded up like a map. The players enjoy drawing in it, writing little notes, and designing the key. And when the campaign eventually ends it will be a nice souvenir for me.
A quick and dirty (though highly effective) way to handle this would be to setup a Roll20 account, create a map and then share the link to the map with your fellow players (make sure they also have accounts!). This enables easy collaboration and the map is persistent so any changes each player makes will show up for every other player too!