Take a look at the API, you have to write a script to update membership for users you want.
// Gets user and membership objects
UserInfo user = UserInfoProvider.GetUserInfo("Username");
MembershipInfo membership = MembershipInfoProvider.GetMembershipInfo("NewMembership", SiteContext.CurrentSiteName);
if ((user != null) && (membership != null))
// Grants the membership to the user
MembershipUserInfoProvider.AddMembershipToUser(membership.MembershipID, user.UserID, DateTime.Now.AddYears(2));