I've missed a raid because I was at work and didn't know one had started.
My suggestion is similar to how there is a glowing red indicator next to the character icon when a character is due for upgrades. Can we possibly have the guild chat icon glow when there is an active raid? Maybe have it be the same color as the status/phase of the raid so that anytime we open the app, we can see that one has already been started.
Not sure if anyone else have missed raids because they didn't know one was on and wasn't communicated well amongst the guild.