Welcome can be used as a noun and as a verb.
As a verb, welcome is transitive, it takes a direct object. This direct object does not take a preposition.
So in both your sentences, we do not use to:
I welcome John.
We welcome “Veeru" junior.
However, when we use welcome as a noun, we need another verb. That verb will have to take a direct and and indirect object. The direct object is the welcome, the indirect object is the person that we welcome:
I extend a warm welcome to John.
We give a warm welcome to “Veeru" junior.
Now, in headlines, or in the case of welcome, on signs or banners, we usually leave out words from a sentence. In this case, it may not be immediately clear whether welcome is a verb or a noun!
If we see a sign that says:
Welcome to “Veeru" junior!
_Welcome _ is certainly used as a noun. The text on the sign is short for we extend a welcome to “Veeru" junior.
Now, if the sign is shorter:
It can actually be read in several ways:
- Welcome can be a verb: [We / I] welcome John.
- Welcome can be a noun: [We / I extend a] welcome [to] John.
It can also be read as an imperative: I order you to welcome John.
How do people understand which one is meant?
They do not. The meanings are so similar (John is welcome, and we want him to feel welcome!) that it doesn't matter whether we say we welcome him, we extend a welcome to him or we urge people to welcome him.