Sure, I’d love to talk about including other hearing impaired people in FIRST!
First, though, I would like to clarify that there is no end-all, be-all answer to what you would need to do to make a hearing impaired person feel comfortable in the team. However, it is crucial that, as a mentor, you connect with them early on and make sure they have everything they need to communicate. The number one thing you can do to help a hearing impaired person inside and outside of robotics, is make sure they know you and the rest of the team cares.
Each person has different ways of dealing with their hearing impairment and the communication barriers that go along with it, and make sure the team knows what they need. For me, I told the team early on that I did not want them to try to change their way of speaking for me, because in most cases it makes it far more difficult to understand, and if I needed them to talk a little more loudly or anything, I would tell them or ask them to repeat themselves. I was fortunate enough to have a longtime friend of mine join with me, so if needed, he was familiar enough with me that we could communicate easily, and the team was small enough that they were al able to get to know me personally and familiarize themselves with it.
One thing to be aware of, though, is that many people avoid speaking up out of embarrassment or other reasons, so make sure they know that it is perfectly acceptable to stop the speaker and ask them to speak more clearly or whatever else they may need. This is part of why it is so important for them to know the team cares about them and that they feel comfortable with who they are working with; because, speaking from experience, it is a whole lot harder for you to self-advocate to a stranger, since a lot of people (me included) do not like drawing attention to their disability. If it is perceived (or if they feel that it is being taken this way) in a negative light, where it can start to feel like an annoying interruption, is when it becomes particularly difficult to draw attention to yourself. This applies to the team as a whole; make sure everyone feels comfortable speaking up for any reason if they feel confused or need something. Communication is the key to creating a safe and comfortable environment for everyone.
Sorry for the long post, but let me know if you have any more questions. Thank you!