From what I know, the Watchtower Society has not registered itself per the Cuban government, however, recent changes have taken place vis a vis the Socialists and the Catholic church, that in turn would allow more religious freedoms for many different groups including the JWs. Their activities are strictly monitored (no door to door or street witnessing, literature gets "smuggled" in from other nations, like my mom did when she visited.)
The Witnesses were allowed to have their first public convention in 1997. There were approximately 15,000 attendees.
My information comes from people who have gone there and/or have relatives there. Things may have changed/not changed recently.
Cuba apparently has improved little its treatment of the nation's approximately 82,000 Jehovah's Witnesses, who previously encountered government harassment due to their religious opposition to military service and participation in pro-government organizations. But Jehovah's Witnesses are still harrased because they are perceived to be opposed to Cuba's revolutionary principle: "Defence of the socialist motherland is every Cuban's greatest honour and highest duty."
Did not find exact data to your questions, but here are some links with info:
U.S. Department of State Cuba Country Report on Human Rights Practices for 1998 Released by the Bureau of Democracy, Human Rights, and Labor, February 26, 1999.
Jehovah's Witnesses, once considered "active religious enemies of the revolution," were allowed quietly to proselytize door-to-door, and they generally were not subjected to overt government harassment.