The guests at Macbeth’s party might have been suspicious that he killed Banquo, but Lady Macbeth convinced them that he was just ill.

Macbeth came to power through illegal means, obviously.  He killed the current king.  Because his friend Banquo knew about the witches’ prophecies and Macbeth’s desire to be king, Banquo had to go.  At the party, Macbeth had a lot of guests who saw his outburst.

MACBETH

The table's full.

LENNOX

Here is a place reserved, sir.

MACBETH

Where?

LENNOX

Here, my good lord. What is't that moves your highness?

MACBETH

Which of you have done this? (Act 3, Scene 4)

Macbeth sees the ghost of Banquo at his spot at the table, and will not sit down.  The other guests of course have no idea what is going on.  They think that Macbeth is losing his mind.  It is a reasonable assumption.  After all, he is clearly seeing things.  It becomes obvious that he is seeing a ghost.  Ross and Lady Macbeth jump in and try to make excuses for him.

ROSS

Gentlemen, rise: his highness is not well.

LADY MACBETH

Sit, worthy friends: my lord is often thus,
And hath been from his youth … (Act 3, Scene 4)

Basically they are saying that Macbeth has fits, and that there is nothing wrong with him that is out of the ordinary.  No, he’s not seeing ghosts!  He just got a little confused.  Macbeth yells at the ghost of Banquo, and this would be disturbing.  They have to tell the guests something. 

Do they buy it?  Lennox retreats and wishes Macbeth better health.  The others leave too.  They probably are embarrassed and do not want Macbeth to realize they are witnesses to his weakness.  Whether it is guilt or madness, they do not want to be involved.  After all, people are ending up dead in the kingdom.  First Duncan died, and then Banquo.  It does not take a genius to know that something is up and all is not well with Macbeth.