The base word is "minum".
Verbs are not conjugated.
Singular/plural/present tense/past tense , it's all the same.
Conjugation doesn't exist in Indonesian. That's one of the good part of the language :-)
They are basically have the same meaning. But we rarely use meminum (affix version of minum) when we speak. We use minum. Meminum is usually used in written language /formal contexts
And also. I asked this question a long time ago, and I've been told that when you have a complement, it's ok to use "meminum", it's more formal.
Saya meminum susu.
But without a complement, it's better to say "Saya minum".