Summon a Raid Party
The best place to summon new Members for a Raid is in the Guild HQ #who-is-available channel by tagging the roles in question, or canvassing the weekly Raider Round up meetings.
Please give a realistic expectation of expected hours, a generalized overview on the Raid (including the client name and project), and budget. The more detail you can provide, the easier it will be to find a Raid Party.
Outside of #who-is-available you may also look in #general or scour active Raids to find Members which best suit your needs.
If all else fails, please find a Monk to help you establish a Raid Party, and be sure to draw from Apprentices to fill gaps as needed.
DO NOT START A RAID IF YOU DO NOT HAVE THE PROPER RAID PARTY IN PLACE.