Geraniums are a good option for ground cover around roses. First, the wide range of geranium color allows you to pick a color that will accent or complement the color of your roses. Second, the growing conditions that these plants like are very similar to those that most rosebushes do well in. This makes caring for the combination flowerbed fairly straightforward and simple.
Plants from the onion family such as alliums and chives make a good ground cover around roses. Not only do they aesthetically make the area look better, but they also play a part in preventing problems that some roses have. For instance, plants from these families keep aphids, an insect often attracted to roses, away from the area. You can also repel fungus as a result of planting members of this family around roses.
If you'd like to make your ground cover more than just pretty, but practical as well, you could delve into growing strawberries. Strawberries provide a lush, thick ground cover once established. Because strawberries produce well for several years, depending on the variety, you also don't have to worry about replanting new ground cover each year. Edible varieties of strawberries work perfectly well as rose ground cover, so you get the added benefit of a berry crop during the season.
Ultimately, the best way to create ground cover for roses is to under seed a variety of plants. That way, you can plant seeds that flower at different points throughout the year and keep a colorful ground cover all season long. Also, you can plant different types of plants that provide different benefits to the roses. Herbaceous perennials keep fungus away from the roses, while plants rich in nectar attract bugs that eat aphids.