Then go to studio > pages > and choose either your homepage, profile page or your dashboard and look for the invite block. If your invite block is visible to you, then cool, but if not you can click on add block and look for it then add it to your desired page. If you see the block, make sure you make it visible to your desired membership level, see picture please
I saw invitation in the app market, it is not free, 50 $ per year.
Some of the UNA Apps are free and some are paid - many of the paid ones are included in the UNA Pro Plan - are you on the UNA Pro Plan? The outline of the inclusions in each plan are covered here: https://una.io/page/start