I invite myself to meetings all the time, practice group meetings in particular. In our law firm we use Outlook scheduling, at least we try, to reduce the number of “is everyone free for a meeting at X time” emails and phone calls.
When you are meeting with folks outside your organization, you may want to try Doodle for the same effect. Doodle will let you propose meeting times, capture the preferred availability of people you would like to attend, and then assist with scheduling.
Check it out at http://www.doodle.com/.
Are you using Doodle?