They’re both grammatically correct. The situations in which they’re used are different though.

I would use “It is a pleasure meeting you.” or “It is a pleasure to meet you.” at the beginning of when seeing someone for the first time.

And I use “It was a pleasure meeting you” or “It was a pleasure to meet you.” when I’m about to leave.

When to use which of your example sentences is all up to what tense the verb is in. If the verb is in the present tense, it’s not going to be used when the event is almost over or after it is finished. When the verb is in the past tense, it’s not going to be used at the beginning of the event because you’re not done with talking with that person. I personally think using either of them during the conversation might be a bit strange.

From the second time I see that same person, I would use the verb “see” rather than “meet” as in “(It was very) nice seeing you!” only at the end of the conversation, and a reply to this would be something like “(It was very nice seeing) you, too!”.