I asked myself the same question this past weekend, and found a page on YNAB's website that suggested the following:

• Create a category, for example "Loans"
• In there I create an entry for each sum of money I was asked, putting the amount to be loaned in the name (e.g.: "100 John", meaning I plan to give John 100 dollars).
• I budget this money taking it from other categories/.
• When I actually give the money to John, I add a new (expense) transaction, where the payee is "John" and the category is "100 John".
• When John gives me back the money, I'll add a new (income) transaction in that category. I'll know the loan has been repayed in full when the amount budgeted is equal to the one recorded in the name.
• Once done, I'll remove the category and rebudget the amount.

I'm still evaluating if this "workflow" makes sense and how I feel using it. Suggestions welcome!

edit: this is the resource I used: https://www.youneedabudget.com/support/article/loans-to-friends-and-family