I feel he has a serious job he can't leave? One of the lesser druids would be good though? Maybe that one who talks to the bird.
Putting this in spoilers in case you haven't finished the EA content yet.
After the party, Halsin selects someone else to take his place and joins the pc's camp.
Thanks, I'd forgotten that. So yeah! wouldn't he be pretty high level? But yeah if he's in the camp he should be in the party. Like not a solid rule, litch and all, but definitely for people like him who would help and have a clear adventuring class.