A "Seed" or "Seed Person" is one of two things or possibly both:

1. They are the founder of the BUD
2. They are the spokesperson and point of contact for that BUD

So a person who is beginning a group or what we call a BUD is a Seed Team and they remain a Seed Team until their team exceeds 4 people. Once they are at 5 they become a BUD.

See also:

What Does It Mean to be a "Seed" or "Seed Person" of a BUD?

What is a BUD?