I made a similar post in the wizard forum. Here I will add rationalization for why I may not accept other classes aside from "well, I did want a guild of wizards," because this isn't the wizard forum.
This guild is for Wizards who like to Role Playing, but there are no Role Play rules. The guild name is 'en Templa Arator, which means Champions of Magic, or in magic champion... but the literary translation is Champions of Magic. More accurately, those who champion the arcane arts. Precisely, this means Wizardry. There are two other classes that also always use arcane arts regardless of their direction, sorcerer and bard, but they are not "champions" of magic like Wizards are. Socerers have a limited knowledge of spells and can use every arcane wand. Bards are specialists that posses minor arcane knowledge to assist their allies.
'en Templa Arator will initially start off with wizards only. So if you're on Thelanis and have a pure wizard, ask Uthala to join. I will accept multiclasses later on, but not at the moment. The first that will probably be allowed in are Arcane Tricksters... but even that will take time. So within a few months of this posting, only pure wizards for now.
Arcane Archers and Melee Focusing Wizards will not be allowed in immediately becaue we're looking for wizards that constantly hone their arcane knowledge. Using every opportunity to practice anything otherwise means you're not quite as dedicated to arcane knowledge as the guild would like you to be. Perhaps it will be expanded, but for now it's not.
That reasoning is oddly enough why Arcane Tricksters might be accepted relatively soon. That is, Wizards with a splash or rogue, able to cast high level spells, and effectively. As long as you're not some melee, you'll be welcomed later on... but not for now.
Sorcerers and Bards will not be allowed in because they have a limited set of knowledge in the arcane compared to a Wizard. I will admit that they know more than all the mother classes, in terms of RP, but not nearly as much as a wizard does. I have a vision for a scholarly type guild of wizardry. Maybe they can fit in a role later on.