In multiplayer mode, there is a great chance of gaining more gems than you would by yourself in single-player, but it all comes down to two things – communication and the size of your clan.
The rewards are greater, but then again the rewards will still be there even if you lose, for bonus loot, but reduced by a lot.
Keep in mind that in a clan war, you will need to co-ordinate you attacks for maximum efficiency to gain those stars, since even if you gain full three stars on your attack but your other 9 clan mates gain only 1 or less in their attacks, you are likely to lose.
Just remember that victory is calculated on which clan gained the most stars collectively in their attacks, and not individually.
When joining a clan, you should look out for several things.
Firstly, the size of the clan. Smaller clans tend to be less able to aid each other with donated troops times of battle compared to the larger available pool of donated units by larger clans.
A larger clan is always more preferential, but only if they follow the other requirements below.
Secondly, level difference and variance.
If your clan has a lot of people and all of them are 3 to 6 levels higher than you, there is little incentive to fight other clans around your level, which means much less loot for you, since dyou won’t have enough high level units or defences to defend against or attack similarly higher level units sent by your enemies.
Thirdly, their battle history and win streak.
Obviously, you won’t want to join a clan with a lousy win streak, since that’s an indicator it simply can’t win much.
Battle history will tell you how often they fight and when they do.
Fourthly, and very importantly, communication and time zone differences.
Everything can be good and all, but if no one talks to each other and no one coordinates attacks in clan wars, or if you’re asleep whenever your clan is in a clan war then good luck – you’re not going to get much done, or be able to receive or give troops to your clan.
With that, good luck, and go raid some warbases!