When I tested this earlier in the year I could customise Sharepoint forms with Power Apps designer and all users could use the new form as long as they had a Sharepoint licence. (They did not need a Power Apps licence)
When I try today users seems to need a Power Apps licence just to fill in & submit a custom form, is this correct or does it depend on certain features being used in the form ?
Endless Googling I cant see anything offical from MS on this either way, apart from stating that External users DO NOT need a PA licence.
I can confirm that you need a Power Apps licence to view customised SharePoint forms in Power Apps, so sharing with external users may send you back to InfoPath.
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