
Does it mean
1) the table we already know or at where are sitting some other people
2) just one table for all people
?

It depends whether the speaker is talking to one person or more than one. If said to one person, it means that the speaker wants himself and the person he is talking to to sit at the same table. It could also be said to more than one person, in which case the speaker wants the people he is addressing to sit at the same table as him (the speaker). In both cases, there may or may not be other people also sitting at the table - this is not clear from the sentence.
Because "table" is in the singular only one table is meant in this sentence.

Could it also mean let's sit at the same table as we used to sit before?

Yes, it could mean that. As it is a short sentence it is open to interpretation, and context would be required to be sure what it meant.
It could be "Let's sit at the same table [we sat at last week/yesterday etc] with [] understood/not stated.

Thanks a lot for this explanation!