Users can Participate in online meetings if they are not a part of your organization or if they don't have Lync installed as they can utilize the Lync Web App for this.
Check Your Meeting Readiness
Before meeting time, run the meeting readiness program to determine if you have the programs you’ll need for a successful Lync meeting. You may have to download the Microsoft® Silverlight® 4.0 browser plug-in, or a later version of Silverlight, and then restart your computer.
To run the meeting readiness program:
• Open your Online Meeting email invitation, click First online meeting? at the end of the invitation, and then follow the instructions.
Join the Meeting
To join the meeting, do the following:
1. At meeting time, open your Online Meeting email invitation, and then click Join online meeting in the email message.
2. In the Lync 2010 screen that opens, click Join the meeting using your web browser.
3. If you have an account with the organization that called the meeting, click Join using your corporate credentials.
NOTE: With corporate credentials, you will have a few more privileges, such as being able to join the meeting without waiting for the leader to approve you.
Otherwise, click Join as a guest.
4. If necessary, enter your corporate credentials or type a display name for yourself, and then click Join Meeting. You may have to wait until the meeting leader lets you in, or you may be admitted right away.