It depends on the context. Commonly it means something like "I agree with you, I have had exactly the same experience", this is an ironic use, the ironic meaning being "you don't have to tell me, I already know". However, it could also be 1 - if your friend did not already know all about it and had not had the same experience and wanted to know more. If the friend did want to know more then "tell me about it" would be said with a questioning intonation, if the first option the intonation would be different, maybe flat or ironic.

Often means: I know how that feels - I've been in a similar situation myself (In fact you don't really want to know more about it! - It is all too familiar...) It could mean No 1 too - it depends on the context... "My wife/ husband spends so much money - we never have any left at the end of the month!" "TELL me about it! My husband/ wife does exactly the same thing!"

"I'm so tired of all the rain!"

Friend: Tell me about it..🙄😒

Or

"I'm having a bad day" 😓

Friend: ugh tell me about it

They are sharing the same feeling as you. It means "I agree!" "Yes me too" & "you're not the only one"

Its said in a flat tone.. "tell me about it 😑"

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If your friend however says it as a question

"I went on a trip"

Friend: "oh! tell me about it(the trip) " 😃

Its normally the first. Only unless they ask it as a question. Listen to the tone of voice

He wants to know more.

it means that you have to talk to about yourself for example : mi name is... i live in ... i like ....