Unfortunately, there's little that can be done retroactively and we are not able to restore members to a guild they were kicked from.
Players with higher guild rank have the authority to kick out players below their rank. It can be very frustrating if players abuse their authority, but there aren't any rules against this. The best approach is to restrict who you let into your guild and promote with cautions.
Also, consider adding members that you like to other social media, like Twitter, Facebook, Game Center, WeChat, etc..., so that you can easily find them again if you get separated.