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.
Please give a realistic expectation of expected hours, a generalized overview on the Raid (including the client name + 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 suite 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.