Cambridge City Hall will host a special session of voter registration tonight from 6 to 8 p.m. as part of a Greater Boston voter registration drive coordinated by the National Movement for the Student Vote, Inc.

The special session is designed to afford new voters between the ages of 18 and 24 an opportunity to register at other than the usual registration hours of 8:30 a.m. to 4:30 p.m. at the Cambridge Police building.

Few Students Registered

According to a spokesman for the Student Vote, Inc., a Washington-based foundation whose aim is to involve students in the registration of new voters, only 15 per cent of Cambridge's 18- to 21-year-old age group are already registered. The organization hopes to register at least 100 more such voters tonight, working in cooperation with the League of Women Voters and the Cambridge Elections Board.

This special registration opportunity is part of a Greater Boston drive scheduled to last until March 25 which will encompass all 42 towns in the Greater Boston telephone directory.

Rhode Island Drive

A week-long registration dirve recently completed in Rhode Island under the sponsorship of The Student Vote resulted in a 25 per cent increase in the number of 18- to 21-year-old voters there at a cost to that organization of only 11 cents per registrant. The Democratic National Committee had previously cited $2 per voter as the minimum figure possible.