If Lord Capulet has already planned a party and not yet invited Paris to it until he sees him, this invitation most probably is given as an afterthought. Capulet probably feels awkward in this situation, realizing his social faux pas of not having invited some one of the prestige that Paris possesses and his being held in a bad light after the street altercation. That he has not wished to have Paris at the party for Juliet is evidenced in Lord Capulet's dilatory invitation and in his protestations to Paris's request that he be permitted to marry Juliet despite his politic remarks,
My will to her consent is but a part;
An she agree, within her scope of choice
Lies my consent and fair according voice. (1.2.17-19)
Certainly, it is apparent that Capulet is fauning because he cautions Tybalt against doing anything after having espied Romeo, their mortal enemy:
He shall be endured.
What, goodman boy I say he sahll Go to,
Am I the master here or you? Go to.
You'll make a mutiny among my guests! (1.5.80-83)
Most likely, Capulet has not invited Paris before the evening of the party because he does not want Juliet to fall in love with him and marry Paris. Later in the play, Lord Capulet succumbs to the pressure of Paris and he also seeks a way to cure Juliet of her depression after Tybalt has been slain so, despite his better judgment, he agrees to have Elizabeth marry.
I would say that this suggests at least a couple of things to us:
- I think that it suggests that Lord Capulet really does want Paris to end up marrying Juliet. Paris is related to the Prince and so he would be a good "catch." If Capulet invites him to the party that's already been planned, it shows that he wants Paris to think well of him.
- I think it suggests that Lord Capulet is quite rich. He's throwing a big party and his attitude is pretty much "the more the merrier." If he doesn't have to worry about how many people show up, he's probably pretty well-off.