Here's why:
* "Group" should be plural: If you're referring to multiple groups, you need to use the plural form "groups."
* "Too" is not always appropriate: "Too" implies that something is in addition to something else. It might be better to rephrase the sentence depending on the context.
Here are some corrected versions depending on the intended meaning:
* "Definitely join these groups." (Simple statement)
* "Definitely join these groups as well." (If there's a previous context about other groups)
* "Definitely join these groups too, if you have time." (Adding a condition)
Let me know the context, and I can help you write a more accurate sentence!