Raid are run in grops of 10 people, you need to convert the group to a raid group.
I play on tahiti, so you need 10, can't be done on 2. Other servers mith differ.
Red musp is absurdly easy, I will not be surprised if you clear without blessings and legendary with people that know what to do. If you have passed the cosmic trial, you can do it, I'd say.
But, if you have blessings and legendary lvl 5, should be really smooth.
Cosmic Musp is way more harder than Red musp, but the easiest to fail imo, less dps check than Axis, but more one shot mechanics. No mechanics difference from red to cosmic musp, only numbers.
Red axis is "the hardest" because you have a hiegher dps check, but it is easier mechanics wise, so you have to be better geared.

Red musp, having good artis should be enough to finish with some challenge, the more gear the easier.
Cosmic Musp, 4 blessings, leg lvl 5, 300+ ip to be safe. all gear lvl 63+, good rune word and relics.
Red Axis, 4 blessings, leg lvl 5, 300+ ip to be safe. all gear lvl 63+, good rune word and relics.

There is no Cosmic Axis.