There are only two ways to do it.

Option 1:
You could just post an update on your feed saying, "hey everyone I'm headed to [conference] on [date] in [location]. Let me know if you're around and we can meet up!"
That will at least get to people that check in on Linkedin occasionally.

Option 2:
The way your doing it now (manually)

Linkedin made it impossible to message multiple people at a time without manually adding their names (or clicking each person), one by one. You can't paste in a list of names like you can in email. You have to put in a single name, click the profile that pops up, then put in the nest name, etc.