http://dawnspire.tinyrocket.se/index.php#challengeGuild&id=88 (pressing this link will automatically create the challenge, so remember to remove it)Save the link somewhere for future use

Edit. So basically what I just did was to copy the link, for example, from challenging UA, and then just changed the guild id to match ours. You probably need to be logged in to make it work... Also make sure you play them as practise matches. You shouldn't get any rating from it anyways, but just to prevent messing up the system.