Anne's Brainteaser

Marie's party is an evident success. Dozens of guests sitting around four tables share the joy and good humour of the celebration, while savouring the tasty food and refreshing drinks provided by the hostess.
At her table, Anne looks at each one with a quirky smile and says "I bet that there is an 80% probability that two of us at this table have the same zodiac sign!"
The others look at her incredulously: "How could she ever know?"
Aware of the scepticism, she takes the challenge a bit farther. Glancing around the room, she pronounces calmly "...and I reckon that there is a 95% probability that two persons in this room share the same birthday!"

Anne has got a reputation for having a sharp mind. For her to feel confident in stating such probability values, how many people do you figure were sitting at her table, and how many guests were there in the room altogether?