Go to the Mailbox, usually found by an inn, and right click. Click on the SEND tab at the bottom. Put Moneybags in the TO section, put "For Tabard" in the subject field, write a word or two in the text section, and put the amount in at the bottom (e.g. 50 in the silver box). Click on send. You will be asked to confirm the amount, so just click OK.
You can, of course, use this method to communicate with any player, including any ALT characters you have. If you are sending an item you drag it to the item slot - and you do not have to write anything then in either subject or text field.
You will probably have chatted with the prospewctive member and got his/her agreement first. Open the Friends/Guild/.Who window (by typing O or clicking on the icon at the bottom). Select the GUILD pane. Click on ADD MEMBER. A dialogue box opens up - type the name of the character you are inviting and click accept. An invitation is then sent to that character who has the option of accepting or not - you'll know if they accept when a message in yellow appears.
Let Golgamesh know who you have recruited and whether you have promised any free gift (eg bag) which should be sent to the new member.