Hi @Srinivasa K
Read multiple articles and it refers to enable at organization level and not possible at the individual users.
Yes, you're right! Teams federation is enabled at organization level. If you want your organization to chat with individual user. To invite them as a guest in your Teams is a good choice for you. With guest access, you can provide access to teams, documents in channels, resources, chats, and applications to people outside your organization.
For more details about how to invite user as a guest, you could refer to:
https://learn.microsoft.com/en-us/microsoftteams/add-guests?msclkid=195ccc17d0fd11ecb53baf77b4b6bf02
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