Yes this is appropriate! Many if not all photographers do this starting out - it’s called trade work. Just let them know up front that you’re currently a hobbyist looking to learn and build your portfolio (it’s not too late, you can just shoot them a reply with like a date and time confirmation, thank you, and your “disclaimer”). Offer free unedited digital jpegs in exchange for their time (don’t offer actual raw files), and best case scenario is that they love the photos, post them on Instagram, and tag you. Worst case is that they don’t like them and just don’t share them - they’re not wasting any money. Either way, you’re gaining experience in a really fun way.

I’d echo what someone else said about getting a signed model release from each person you’re photographing, though, if you plan to actually use the photos in your portfolio or post them on social media. You can print off releases on paper and have them sign in person, or use Adobe fill and sign to send them via email.

You never need an excuse to make art and collaborate with people :)