Welcome to the community Dawid!
So…
Yes, they can, using Monzo.me or ideally (since Monzo.me costs Monzo money) by bank transfer / standing order.
Funnily enough, another user tried this today. PayPal did charge conversion fees (having ignored his request to top up the card using foreign currency) but they did refund those fees -
You can withdraw your funds from an ATM of course & Monzo support can sometimes return a deposit to your bank account. But you won’t be able to transfer funds to non-Monzo users from your card (it’s best to get your friends to sign up too 
).
This is a friendly community so please feel free to ask! But as you can see, quite a few of these questions have been answered in other posts so it’s worth searching first. If you can’t find the answer, no worries 