In William Shakespeare's play Romeo and Juliet, what is Capulet's reaction when he finds out Romeo is at the party?

In Act I, Scene V of William Shakespeare’s Romeo and Juliet, Romeo, his cousin Benvolio and his close friend Mercutio sneak into the Capulets' masquerade ball so that Romeo can observe the target of his unrequited affections, Rosaline, a member of the Capulet clan—the family with whom the Montagues maintain a protracted and deadly rivalry. Romeo is deeply in love with Rosaline, despite the fact that she pays him no attention and has vowed to remain chaste. So depressed is Romeo about his failed efforts at wooing Rosaline that he and the other two conspire to crash the Capulets’ ball despite the risks inherent in such an action. As audiences and readers of Shakespeare’s play know from the outset, the rivalry between the two clans is serious, and any encounter between partisans of the two families will invariably turn violent.

Earlier, in Act I, Scene II, so lovesick is Romeo that his cousin Benvolio suggests that they sneak into the Capulets' ball so that Romeo can, by comparison, see for himself that there are many women more beautiful than Rosaline, and that Romeo’s infatuation with Rosaline has skewed his perspective. Benvolio insists that, once Romeo has viewed the other women at the party, he will be quite pleasantly surprised by what he sees, and will no longer pine for Rosaline:

At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lovest,
With all the admired beauties of Verona:
Go thither; and, with unattainted eye,
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.

In Scene IV, the three Montagues are at the Capulets’ ball, exploiting the anonymity provided by the masks worn by all attendees, but it is in Scene V that Romeo first spots Juliet, asking of a servant,

What lady is that, which doth
enrich the hand
Of yonder knight?

And it is now that Tybalt, the short-tempered, belligerent member of the Capulet clan, identifies Romeo, having overheard the interloper’s conversation with the servant:

This, by his voice, should be a Montague.
Fetch me my rapier, boy. What dares the slave
Come hither, cover'd with an antic face,
To fleer and scorn at our solemnity?
Now, by the stock and honour of my kin,
To strike him dead, I hold it not a sin.

Having accurately identified a member of the rival Montague clan, Tybalt is prepared to kill Romeo right there in the middle of the party. He is stopped, however, by the more level-headed patriarch of the Capulet clan, who cautions Tybalt against carrying out any such act amidst the festivities, stating:

Content thee, gentle coz, let him alone;
He bears him like a portly gentleman;
And, to say truth, Verona brags of him
To be a virtuous and well-govern'd youth:
I would not for the wealth of all the town
Here in my house do him disparagement:
Therefore be patient, take no note of him:

The senior Capulet is considerably less hostile towards the Montague interlopers than is his hot-blooded nephew Tybalt, and bear no ill-will towards the young Romeo. To answer the question, then, Capulet’s reaction to Romeo’s uninvited presence at the masquerade ball is to allow the Montague to go in peace. As we know, however, the doomed couple – their fate already suggested in the play’s prologue (“From forth the fatal loins of these two foes / A pair of star-cross'd lovers take their life”) – will fail in their effort at being together in life because of the enduring hostility that divides the two families.