Sorry for using a novel, but the author, Patrick O'Brian, is extremely accurate on all aspects of Navy for the era :

Sea Militia was organised by region (eg "Sea Militia of Norfolk") and provided land appointments with full pay to RN officers who otherwise would have been on half-pay because they could not obtain sea command (because they wanted to stay at home, or where not deemed able for service at sea, or simply because they were two more commanders and captains than available frigates and ships of line).

No payment seem to have been asked, but it was a loss of any possibility to be rewarded for naval actions or trade capture.

The rank and file are not described.