The practice is quite common. Whether using a finger or tongue, circling the outside of the anus is known as "Rimming". The outside of the anus and just inside are richly endowed with sensitive nerve endinding that turn on and become reactive when a person is very highly aroused. In fact, involving the anus when a person is about to peak helps jumpstart an orgasm, either making it more intense, or helping to trigger an elusive one. Common sense and etiquette demands hygene, so please bathe, first.
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For more on the topic, please read this article:
"Playing Ball" and How To Tips for Prostate Massage
This is also an excellent tutorial for people interested in and concerned about the how-to of Anal stimulation as well as play.
I hope this is of help. Got questions?