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When did Bruce Hornsby join Grateful Dead?
Bruce Hornsby played more than 100 shows with the Grateful Dead from 1988 until Jerry Garcia's death in 1995.
How did Bruce Hornsby get involved in the Grateful Dead?
At some shows in 1988 and 1989, Bruce Hornsby joined the band as a special guest and played accordion or synthesizer.

Question: How long did he play with them?
Self-contained rewrite: How long did Bruce Hornsby play with the Grateful Dead?