I wouldn't say there are those types of groups, especially not for Twitter. you can buy followers, it's not hard work, but in reality, you won't gain much from it. Only a lot of inactive accounts and non-interested in your speech.
There are definitely Twitter engagement groups on Discord and Telegram - I know of several mid-sized accounts that use them or have used them in the past. Unfortunately, none of the accounts would give me more detailed information. Probably because none of the accounts want users to know that they have used a paid service for engagement/followers in the past.
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Thanks for the links. The thread is from 2019. Do you know if the service is still active?