Amicable refers to a friendliness or goodwill between people or groups. Amiable refers to one person's friendly disposition. A group might have an amicable meeting, because the people there are amiable.

Amicable comes from the Latin amicabilis, for friendly, which can be traced to the Latin amicus, for friend. Here are some friendly examples:

Through an email, Sala said negotiations have been very amicable and that both sides are motivated to get a deal done. (Seattle Times)

It was a relatively amicable divorce and at least they remain on speaking terms. (BBC)

The split was amicable, Mr. Robbins said, with his former partner breaking even on the sale and returning to the music business. (New York Times)

Amiable also comes from the Late Latin amicabilis, with a stop in Old French as amiable before English adopted it. Some experts use amiable to describe only people, but amiable is also used to describe things being sociable or agreeable, like an amiable tone of voice. It's usually used for people, though:

A tall, amiable man, he wears a white hard hat with "JOE" written in faded marker on the front. (Washington Post)

One former roommate described him as amiable and talkative. (New York Times)

Amicable refers to the connections between people or things, like a friendly situation with a crowd. Take out the "c" for crowd and you're left with amiable, one friendly person.