I have you warn you though that the userlist (as in joins/parts) in Twitch Chat are not very well suited for greeting people as they join. Joins can be delayed by quite a bit, so by the time you think they joined, they may have already left (add stream delay to that and you may be greeting them a minute after they joined).
In addition, people are sometimes shown as having left even though they are still there. You should take the userlist and joins/parts only as a rough indication of who is there.