We're a two-people guild. I'm the guildmaster, and I've set my friend as a co-owner on our guild island, but yet he's not allowed to initiate upgrades for some reason. Why is that? It keeps saying he doesn't have the rights.
Buildings have their own access rights, so you would also need to give him access on the buildings themselves. But given that you set him as island co-owner, he is also simply able to claim buildings and then upgrade them and manage their permissions in general.
In short, give him permissions on buildings or have him claim the buildings.
That fixed it! Thank you.