Getting more members is really it. With i think 10 members, you unlock the guild bank. With 50 you unlock the guild store. That's really it though. Not really anything else to unlock after that, other than possibly claiming a keep in Cyrodiil.

The best thing you can do for your guild would be participating in guild activities and supporting other guild members. Answer questions, be helpful, work on building a community with the rest of the people in your guild.

That said, I do hope that Zenimax adds some other kind of incentive system for guilds, like unlocking some kind of guild property or a unique tabard or badge that you can all wear to identify yourself as a member of a particular guild. Perhaps some kind of perk system, though that may border too closely on the systems implemented by other MMOs and I'm sure some of the purists will crap all over that idea.

@Opioid‌ hit the nail on the head.

I think the ESO guilds hearken back to the older times, where a guild was a small community, and not simply a collection of perks. The best way to contribute to your guild is by being a member of its community.

Personally, my biggest tangible contribution to my guild (I only have one) is that I don't use trade guilds. That way, I can make the items I post on the guild store accessible to my guildmates at prices that I think are pretty low. Since I'm guilded with like-minded people, it stands that none of us really "need" any sort of trade guild. With all of us questing and sharing things for free, or at very low costs, we tend to have everything we need between ourselves. It also keeps all of our gold within the guild as well, so none of us are lining any other guild's pockets.

Other than that, though, just good conversation. I'm a bit of a Chatty Cathy, so it's nice having people to talk to while I'm adventuring around. If I have questions, I can always ask in guildchat, which is a lot easier than asking a newbie question in zone chat... 0% of getting trolled.