Conversation so far:
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.
How long did Bruce Hornsby play with the Grateful Dead?
Bruce Hornsby played more than 100 shows with the Grateful Dead from 1988 until Jerry Garcia's death in 1995.

Question: Who invited him to join the band?
Self-contained rewrite: Who invited Bruce Hornsby to join the Grateful Dead?