Voir (to see) is infinitive form. “Come (you) to see us on Sunday.” In this sentence come is conjugated in the 2nd person singular but voir (see) is the infinitive form exactly like in the English sentence structure.

There are two verbs following one another venir and voir. Only the first one is conjugated, here venir in the imperative. The rule is that the second one must be in the infinitive, you never conjugate it. This can be explained because it is a verb form used as a noun, it has no subject, therefore you cannot conjugate it.
So your sentences should be:

• Viens nous voir dimanche.
• Venez nous voir dimanche.

Be also aware of the sentence structure that is different from English. The direct object pronoun (here nous) goes between the two verbs and not after.

Other examples where the first verb is not in the imperative.

• Nous allons manger.
• Je veux partir.
• Il est allé courir.