Welcome to Gaia! ::

there are 2 methods

one: post their names in the invite topic and ill send out an inite saying you reccomended them

two: ask them to join (ask, beg all the same to me)
link if needed: http://www.gaiaonline.com/guilds/index.php?gmode=index&guild_id=40967
from there its just a meatter of clicking join guild at the top of the screen

it may be just a captain/vice captain thing unless the moderation panel is visable at the top of the fourm (top-left just under guild members)

but the only way Ive found members can invite is by asking via PM or post
(i wouldnt know about crew or VC's cause ive never been in that possion)