On the trail Delorme InReach kinda sorta allows you to do this, but the ability comes with a subscription cost. Also links to FB and the Twitters as desired. Impractical for other times than hiking but the ability to piggyback can mean always being in touch should that be what one wants.

The very last-century Yahoo groups have many specialized lists, such as the John Muir Trail group.

Cheers,
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--Rick