It takes time to organize events, so finding like-minded people to help split up the load is important to make your Sunday Assembly sustainable.
Depending on the size of your group and the needs of your venue, you may want to recruit volunteers for the following roles. (It's common for people to take on multiple roles. Just be careful that you and your fellow volunteers don't take on too much.)
Organizers - A team of people to help brainstorm Assembly ideas and recruit volunteers
Host - Welcomes everyone, introduces Sunday Assembly and the motto, leads the group through the parts of the program, give announcements. May share an address at the end. See "The Role of the Host" on pg 18 of the Start-Up Guide.
Musicians - One or more singers, with or without instrumentalists
Doing Your Best giver
Tech Team - Setup, run, and cleanup any needed A/V equipment
Marketing Team - Could include posting to Meetup, Facebook, Instagram, Youtube, a website, sending a newsletter, and more