Here is how I handle this (and I'll offer an "on-Pre" alternative as well). I do my invites from my computer. For those that must have it on the Pre, there is a method.
First of all, this requires that you have a Google Calendar account. On the web app on the Pre, go to m.google.com/calendar . This will have a link to either a regular Google Calendar web site, or if you use Google Apps, a link to log in there. Once on the Calendar web site (mobile version), create a shortcut so you can more quickly get there in the future.
From the mobile web app, you can create events, and invite users. You can even edit existing events, and invite users.
WebOS 1.4 (supposed to be out this month) may address this, though I haven't heard anything definitive.
Maybe it's because I'm in front of a computer so much, but I really have very little problem with inviting others from the computer, and just using the Pre to track my schedule, and make quick entries (that can be edited later when I'm at a PC).
Hope this helps.