The app offers the same functions regarding guilds as the client or browser version. If you cannot see a guild request, even after logging out/in, please contact our support team at firstname.lastname@example.org
So you have stated that the app is the same as client and browser but you have not answered either of the posters questions.
Does anyone know what the problem is here/how to fix it?
Also is it possible to invite my friends to the guild through the Catan universe app?
If you know the answers then provide them. If not pass to someone in your team who can answer the questions.