You can join the 2 main Edencom supporting ingame coordination channels “EDENCOM Defense Initiative” (more shield focused) and "New-Eden-Defence " (armor focused) to communicate with likeminded Edencom supporter. Joining fleets and helping for a few hours when systems get invaded (yes, you have to grind a lot to really save something, thanks CCP) is the biggest help you can offer.
Both channels have mailing lists with fittings and some general guidelines on what is needed in terms of modules on your ships and what you can expect from the content. EDI recently launched a website to access all these things out of game as well: edencom.space
Joining a faction warfare group is not a good way to support Edencom. Since you will be at war with the opposing factions (Amarr/Caldari against Minmatar/Gallente and vice versa) and you will lose standings to the opposing factions, you cannot go into opposing factions’ space and therefore cannot help Edencom there. You can stay in an NPC corp or join an edencom supporting corp if you want to. As long as you can show up to fleets and fly desired ships with appropriate fittings (the website has most of the info on that and information are being expanded upon regularly), you already help a lot.