Look at it this way, I'm in a 200 million GP guild and this is the first time we bothered to even progress to the Mundi phase so we can try to earn shards. I beat it my first time, and 1 or 2 other people in my shard have too, but we've also got a handful that attempted it and failed.
I'm low man on the roster as far as GP with something like 3.7 million, so being able to join a guild that can even attempt to get to the Mundi mission would take I imagine something close to that as far as raw GP and enough good teams to actually post some points in the TBs to warrant letting you in.
So what's your GP? How many fully reliced teams do you have? Because to finish 1 wave in phase 1 of that TB it requires serious relic levels and mods and the right teams.