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.

I'm pretty sure LinkedIn only allows you to send one message at a time. You used to be able to send to select all contacts in a filtered group but they took that away some time ago.

In addition to posting an update on LinkedIn, you want want to consider creating a LinkedIn Pulse to promote why you are doing an event there. LinkedIn Pulse are typically articles individuals create but if you think in terms of thought leadership, you may attract people on LInkedIn.