[twoodfin]

I'm unfamiliar with the specific rules of Senate decorum, but no, I don't think any other Senators except whoever's presiding are required to be present.

Senators would get very little done if they had to sit in the Senate Chamber any time any one of them had something he or she wanted to say. They're all pretty garrulous.

[unclenoriega]

This is true. The Senate assumes a quorum is present unless someone suggests the absence of a quorum, so as long as there's a designated president, they're good to go. There are several YouTube videos of pro forma sessions where just the presiding senator and staff are present.