Don't neglect older students, though. Many will come to your group because their friends bring them. Group members should feel free to invite their friends to meetings, trips to Washington, D.C., work sessions, and other activities, especially your group's parties. When prospective new members meet your group in a relaxed, informal atmosphere they can begin to see that pro-lifers aren't at all stodgy and rigid old nannies. You are ordinary folks, good ordinary people, and new members should have a chance to get to know you personally.
Parties build group morale and give members a chance to get to know each other better.
Try to get one of your members to agree to host a party in their apartment or house. Christmas is a good time. And make it a fun party, don't just sit around and drink. Another way to attract upperclassmen to your group is to send notices of your activities to other organizations, like the College Republicans or the Philosophy Club, whose members are likely to become interested in your group. Other than the above notes I don't have any special ideas to attract upperclassmen but you should put up posters, and put notices in the campus newspaper on a regular basis informing all prospective new members that they would be welcome at your next meeting.