A person's name in plural form?
I wanted to ask how a person's name becomes plural in Polish and if there are any rules?
There are no rules about that. They are used colloquially and mostly to describe group of men, women of the same name.
Grażyna idzie na zakupy. - single woman
Grażyny idą na zakupy. - two or more of the same name.
Just treat a name like it's a standard noun, for example: Sebastian- Sebastiny Ula- Ule Michał- Michały
You need to be very careful about that. You're right about Ula-Ule. But masculine names obviously are treated as a masculine-personal. It means it's Sebastian-Sebastianowie, Michał-Michałowie. The version you've provided would be non-masculine-personal nouns. But in general it's true - you just apply rules for regular nouns, but you need to remember about complexity of Polish grammar in this regard.
It's true that the absolute correct form has the -owie ending, but you'll hear Sebastiany and Michały a lot more often. In fact, I don't remember the last time I heard someone say Michałowie... Even Polish poets use it, e.g. "Tańcowały dwa Michały" by Julian Tuwim.
It's a bit grammatically incorrect, but it's used. For example I know multiple guys called Marcin, and when they meet, more often than not they're referred to as "trzy Marciny" rather than "trzej Marcinowie", although technically the later form is the correct one. Poles aren't perfect when it comes to their language too :)