Is "decades’ past" possessive?

Do I have the apostrophe right?

No. There is no apostrophe.

There were a few random music friends from decades past there to see her, and she couldn't be any prouder.

Past here basically means "in the past."

No.

It would be if "decades" were a person called Decades, and the few random friends were from his past:

... friends from Decades' past ...

However, "past" here is an adjective modifying the word "decades." So, no apostrophe.

As others have pointed out, an apostrophe can indicate a missing letter ("Able was I, e'er I saw Elba"), or possessiveness ("The apostrophe's proper usage"), but never plurality, although this is a very common mistake.

I would also like to point out that "There were" is past and "she couldn't be" is present.

Personally, I would match the tenses and write

There were a few random music friends from decades' past there to see her, and she couldn't have been any prouder