When writing a speech to welcome a group, the most important guideline is to know your audience and purpose. Who are you talking to? Why are they there?
Knowing your audience is the key to successful public speaking. You want to try to tailor your remarks to the group. Welcome remarks are often fairly short, and should give newcomers a sense of why they are there. If you are welcoming them to your organization, describe your organization. Give them some background, history, and culture to orient them.
Purpose is also important. If you are making opening remarks you might have many purposes. You might want to instill pride, defuse tension, or inform. If you know what your goal is in making your opening remarks, you will be more successful and feel calmer.