Here are two suggestions that I think will add a few interesting features. I'm just throwing it out there, you guys can help reshape the idea maybe?
As players raid one another, they'll be able to check each other's stats by tapping on the info button as can be seen in the sample image below. This can be used to show off your skills or help guild leaders pick the right players for their guilds.
Once you click on the info button, you'll be shown the below screen. Here you will find stats such as how long the player has been playing the game, or maybe when was the player last online, how many times did the player die and etc. You guys can bring in your ideas as to what stats would be informative and essential to gameplay. I only did this as a visual sample.
Notice there's an invite button. This will only appear when a guild leader is viewing this screen. They will have the option to invite the player to their guild if they're impressed with the player's stats.
Once a guild leader invites a player, he/she will receive the invitation in a pop up box as seen below. The receiver will have the option to either accept or reject the offer.
I do wanna mention, that this pop up should not be displayed to the target player if he/she is in the middle of raiding someone, it may mess up their raid. It should only be displayed when the player is in his/her own dungeon and not in the middle of a raid.