This seriously was what I was expecting. When we found out otherwise today when my Guildies and I all gone on together we were highly disappointed.
You need to allow grouping from multiple factions within the same guild OR you need to give that interface to the Group Finder. It simply is not complete without it.
No one is asking for open world grouping, no one is asking for something that shouldn't already be there as a Quality of Life improvement. It can be done. All you have to do is make tweaks to the system you already have. Guilds are for the most part made up of people who like to play together. Cutting people off is ridiculous and counter intuitive.