A Guild Stash is a special stash that can be shared by members of a Guild and is crossing all leagues - even any future ones (same like any personal, account connected stash - for example when you create it at your torment character, you will get it (another one, content is not connected) at hardcore too)! Guilds do not come with a stash when they are created, but the guild leader can purchase Premium Guild Stash Tabs for 50 points each. The leader can also set access permissions for different tiers of members to view, add items and remove items.
Those points must be added by members (only!) at Guild webpage (acessable also by clicking at guild name at your account profile (you must be logged in), then switch to "My transaction" tab and there you can "Give Microtransaction Points to Guild". Then your GuildLeader must (ingame) go to Shop (M shortcut) and there at Guild section he can choose freely to buy any stuff for your guild.
Be noted that buying is not real time operation - your stash will appear ingame with some lag (maybe even hours).