Re: What would you say to a new student?
Maybe instead of "congratulations," (which I think of as more something you'd say after someone passed their dan ranking...), just saying "Welcome!" (with the exclamation point to emphasize how happy you collectively are to bring this new student into the dojo) would be a very ... well, ... welcoming first line.
Welcome! We at the (YOUR DOJO NAME HERE) are (happy/pleased/delighted) that you have joined us to begin your journey in Aikido. The path can be lifelong if you choose, and we hope that you will enjoy the challenge, growth and joy that Aikido training offers.
If at any time you have questions about your practice, progress, dojo etiquette, or any aspect of Aikido, remember that we are a very supportive community here. Ask! Your sensei and sempai once stood where you now stand, at the threshold of the art, and we are happy to share what we have learned.
(Here's where you can sneakily slip in a line about what the new student can do to help out...):
Likewise, we hope that you will soon feel at home here. In fact, we encourage all new members to take pride and have a stake in their school, as we do. Please feel free to ask what you can do to help out around the dojo. Trust us, it will be appreciated, and you will feel good!
Just some off-the-top of my head, but I think the main point is to keep it light and friendly in the Welcome section... all the "meat and potatoes" serious stuff comes after in the body of the material.