All members can act on behalf of a business (school/organisation/club) in Signmee.
First, you must be invited to join the account as a member. Once you accept the invite and link your personal account to the business, you can act on behalf of the business.
It is important to note that members are assigned roles and privileges. Some members may be able to create communications on behalf of the business, but may not be able to send. For more information on member roles view Roles (Member) knowledge base article.