Hi Charity,
There are a number of Scriptures that they use to support their preaching (some which i remember):
Matt 24:14, 28:19,20; Romans 10:9, 1 Cor 9:16, Heb 13:15, Luke 24:47, etc.
Regarding who would do the preaching work, they believe that the NT was primarily written for the anointed, so the anointed would be forefront in the work. However, since John the baptizer, Noah and other prophets of old did evangelizing work (to some extent) and since these are believed to have the earthly hope, those of the great crowd are encouraged to support the anointed ones in their preaching activity.
They also use Rev 22:17 to support why anointed and great crowd, both have to do the preaching work:
And the spirit and the bride+ keep on saying, “Come!” and let anyone hearing say, “Come!” and let anyone thirsting come;+ let anyone who wishes take life’s water free.+