The meeting may be held at a time when you are the busiest and even though you have something to contribute, you have other priorities. This is also a relevant reason to refuse to participate. You may, however, promise to write down several key points that you would like to mention and pass the list to the meeting's manager.
Imagine there are five points to be discussed at a meeting and only one of them applies to you. It is legitimate to excuse yourself and say that you will only participate in the part that is relevant to you.
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Article source Inc.com - a U.S. magazine and web focused on starting businesses