When to offer co-authorship vs asking for feedback I am quite new to this game and I am not sure how to ask people to be co-authors on a paper I have mostly written myself but that I think would get better  with someone else's input. When is a good strategy to offer a coauthorship and when should I simply ask for a feedback on the paper? This is somewhat of a review paper and I feel that it would be important to have some experts from the field. But regardless how should I decide when to offer a coauthorship?
I'd ask for help. If said help turns out relevant, add a footnote thanking for it. If the help is in some way crucial (found and fixed a fatal flaw, suggested a (better) way to compute the results, suggested a relevant extension, ...) ask the helper if they want to be coauthors or just be thanked as above.