How do I command attention without seeming arrogant?
Be clear, calm, and considerate. Wait until it's your turn to speak, be succinct, and never interrupt. Sit up straight, make eye contact, but only for as long as seems appropriate. It takes respect to convey genuine confidence, but it is the easiest way to command attention. Join in the conversation instead of trying to dominate it.
How to get noticed without overpowering people
They are more likely to listen if they know what you're saying. Get straight to the point, don't go in a long way around. Use a steady voice, complete your sentences and stop apologizing for all that you say.
It's also helpful to know when to talk. Let someone else go first, and then share an observation, question, or idea that builds on what they said. While interrupting, talking over others, or making everything about you may be a way to get noticed, it might damage others' trust in you.
Rather than attempting to flaunt your rank, you can act confident by behaving calmly. Hold yourself in an upright posture, avoid jerky movements, and meet others' eyes. Don't feel that you must be the most talkative person in the group. Sometimes a brief remark at the perfect moment will do the trick better than talking a lot.
Finally, your confidence should make room for an interest in others. Ask them their opinions, acknowledge the value of what they say, and give credit when an idea succeeds. And if they disagree, react to what they said rather than challenging their argument.
Before you open your mouth, ask yourself what you want the group to know or do. This will keep you focused and relieve you of the pressure to perform. Attention earned through clarity, preparation, and respect tends to hold out longer than attention garnered through domination.