"
We have 2-3 groups of shrub roses.... How do I prune/thin out these bushes.

When should I do this. I have no experience with other roses.

Not now. Wait until late fall or winter. Anytime before leaves are
developing (just buds are visible) is a fine time to prune.

BTW, with most shrub roses, you can encourage re-blooming by cutting
the plants back *lightly* (just enough to remove spent blooms) after
the first bloom.

Lynne in Orange County