first u have to finish the main story. then later I'm pretty sure after u let Manaphy back to the sea u go to the guild and no one is on the floor with the job boards so u go to the bottom floor and Team Charm is there surrounded by the guild members they talk awhile to Wigglytuff then talk about Aegis Cave then ur partner will ask them if u can join them. then u team charm and the WHOLE guild go to Aegis Cave
your leader has to beat them,then they will ask to join your team (in aegis cave)
if you mean on the dungeons list AEGIS CAVE is after miracle sea..veronica
In Aegis Cave
You have to go to Aegis Cave, But to get there you need to release manaphy into the sea and sleep for a few days and then team charm comes and takes you to Aegis Cave.
Do a few jobs, and after a while a Mr. Mime will show up by the Kecleon Shop in Town. Talk to him to continue your adventure to Aegis Cave.
aegis cave
Kill an unown that's shaped like an R
You have to get the stones that the unknown drop and use them at the stone marker.Like at the begining you have to get the unknown stones that spell ice.
You gotta defeat Unowns in the aegis cave
Just head back through Aegis Cave and fight him again. There's a chance of recruiting him whenever you fight him.
aegis cave at level 49, along with regirock, regice, and registeel.
Quicksand Cave.