"Want to grab a bar stool sometime?"
OR
"Want to grab a bar stool sometimes?"
And the second one ?If you're asking "Would you like to go and have a drink at some point in the future?" then your first sentence is correct.
And the second one ?
The second one isn't very natural English. You don't normally ask someone if they would like to do something sometimes. You might ask them if they do something sometimes.
Do you go to that bar sometimes?
Do you eat at restaurants sometimes?
Sometime refers to a single action in the future? It does refer to a single point in the future, yes, although of course it's possible that the action might be repeated. If you said "Do you fancy a game of tennis sometime?" then that refers to one single game, but of course it may lead to playing more often.
Sometimes more like on a regular basis? Yes. Sometimes is a description of regularity, like "always", "never", "often", "occasionally" etc.
See above.
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