Maryland has no licensing or registration requirement for process servers:
Rule 2-123. Process-By whom served
Service of process may be made by a sheriff or, except as otherwise provided in this Rule, by a competent private person, 18 years of age or older, including an attorney of record, but not by a party to the action.
All process requiring execution other than delivery, mailing, or publication shall be executed by the sheriff of the county where execution takes place, unless the court orders otherwise.
For more information, visit the Maryland Courts website, or the Mid Atlantic Association of Professional Process Servers.