Right; so here's some basic strategy for Raids I've found successful enough to be clearing 5* raids pretty consistently with only 2 people, as well as some general rules regarding Raids.
1: Since Dynamax rotates between players 1-2-3-4; Player 1 [The host] should NOT use Eternatus/Zacian/Zamazenta, so that they can Dynamax Turn 1. Also; it is generally better to have a pokemon slower than those three; because MAX moves take off 2 barriers; so your Dynamax can hit Barriers than spawn at 25% or lower
2: Players who are not hosts can bring Zacian/Zamazenta/Eternatus, since you're not delaying Dynamax shieldbusting.
3: You can also play a more supportive role:
a: Weather-induceing is quite useful; especially against raids weak to Water and Fire. I personally frequently bring my Drought Torkoal to Raids between it's durabilty and support.
b: Likewise; debuffing moves and abilities can be useful. Especially on a fast pokemon who can apply an ATK/Sp.Atk debuff before shields go up.
c: Reflect/Light Screen are huge, and can help AI die less; my go-to is a Xatu with Duel Screens.
d: Do NOT bring self-buffing moves however, the Raid will frequently dispel your stat changes.
e: Fast [Or Prankster] Taunt can be useful Turn 1; some Raids set-up and will start oneshotting everything. Shell Smash raids are particularly scary due to barriers.
f: When up against a Raid that may know a spread move [IE: Water/Ground/Rock] consider bringing a pokemon with Wide Guard. Especially with NPC's. Or consider bringing a pokemon that resists or is even immune to such moves.
g: Remember status moves won't work on barriers; neither will secondary effects trigger.
4: Do not bring moves such as Earthquake that hit all pokemon on the field. They will end up teamkilling. This generally makes Ground-types pretty bad in Raids [Unless they are special attackers with Earth Power]
5: Since you can only die a limited number of times, and the mechanics of the barriers [Raids will be ending with -4 DEF at 5*]; tankier pokemon >>> glass cannons. Also remember you can use items to heal! [I once got Frozen for example...] Fainting not only adds to the KO counter but reduces how much barrier damage you can do; making more fainting more likly!
6: Avoid the Swoobat raids. They know Future Sight which is seemingly bugged in Raids and causes you to crash.