I started a guild up with just my brother in hopes of getting the 100 slot bank and splitting it.. Ugghhh has it been a long process.
You have to build up influence and use those points. First you need the Lvl 1 upgrade which is 500 IP. Then you need the LVL 2 Upgrade which is 1000 IP... Finally.. with another 1000 IP you can purchase the Guild bank..
Don't give up on the bank, just keep logging on and doing things, if you can do them with a guild mate the rewards are 10 fold. My self, my husband and one friend have a guild between us, which we started when GW2 came out properly in September and we have just managed to get the guild bank. I admit to being an altoholic which helps, every character I log on with gives influence, as long as you actually do something, a heart, an event etc, not just in and out. Keep going with your guild, you will get there.