Your wife may apply for an L-2 visa based on your approved L-1 petition. You need not accompany her to her interview, but she will have to submit evidence of your marriage and your L-1 status at her interview and with her application. Her L-2 visa will have the same validity period as your current L-1 visa.
Seeta L. Nangia, Esq.
Law Offices of Seeta Nangia
Exclusively Practicing Immigration Law
Phone: (415) 273-9123