This is likely A Call to Arms by Alan Dean Foster.
The Amplitur searched the universe for intelligent species to join
them in the Amplitur Purpose. But the Weave, a union of races, fought
for thousands of years to stay independent. Now a Weave scouting party
had discovered Earth, and humanity had to pick sides, for the Amplitur
were close behind.
A black fisherman is one of the book's recurring characters;
”For us to properly determine if you can be of real assistance to us
will involve a journey outside your own solar system. You will be well
treated, properly cared for, and amply supplied upon your safe return
home with the gold you value so highly. No harm will come to you, and
you will have experienced things your kind has not yet imagined.”
Showing how much he had learned, he added with visible effort, ”We
would not force any of you to do this against your will. If you choose
not to accompany us, you may depart whenever you wish.” The fisherman
picked up his simple pack and strode wordlessly forward, followed by
the Australian students and the old woman. The others followed en
masse. Will inhaled deeply, took a last look at the familiar night
sky, put the keys to the van on the driver’s seat, and strode up the
ramp into the hovering transport.