Hi trooper, welcome to the forums.
I assume you are talking about wanting to join a guild.
You do have to wait for someone to send you an invitation, but there's nothing to stop you sending a message asking for one
If you already know of a guild on your world you would like to join, send one of their members a message asking if you can be sent an invitation to join. Otherwise, click on the global ranking laurel leaves top left corner of your city screen, then select 'guilds' tab and visit several guilds' information pages to read about them. There may be requirements you must fulfil before you can join. The info page should also tell you who to contact if you want an invitation.
Hope this helps.
Alternatively you can set up your own guild - but that's another story!