You can gain reputation from any of the following:
- by completing alliance quests, each quest will give points depending on your level and alliance level;
- by participating in alliance wars you will get points based on whether you win (more) or lose (less) and the alliance level of the oposition; these points are awarded even if there is no fight;
- if your alliance owns frozen ossuary you can claim a reputation card worth 800 reputation points each day for so long as you own that land;
- sometimes Aeria or the devs will hold events that reward additional reputation, so keep an eye out for forums.