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To invest in Steem, is it necessary to Power up in a specific community? Or how to do it? Excuse my ignorance on the subject as I have little knowledge on the subject I am new to this digital world.

Another question I have to ask is how much it is advisable to invest? And the other would be if I earn in a post an example 5 SBD so to speak should I invest half to invest in Steem? Clarify that my friend @dobartim please.

You can invest as much as you want, the more the better. Investments are personal, there is no connection to the community, it is your choice.
When you are not an investor, when you get support from a Steem curator, then the rule is that they would vote for you to give 50% of your earnings to Steem Power, and 50% that you can take for yourself and get out of your wallet.

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