Learn Dutch: Deze groep is heel sterk. - This group is very strong.

In Dutch, the word for 'this' depends on the noun. Because groep is a de-word (de groep), we use deze. If it were a het-word, we would use dit (like dit boek).

Could I use erg instead of heel here?

Yes, absolutely! Both heel sterk and erg sterk mean 'very strong' and are perfectly natural in this sentence.

Why doesn't the adjective sterk get an -e ending?

Adjectives usually only get an -e added to them when they come directly before the noun (like de sterke groep). Since sterk comes after the verb is, it stays in its basic dictionary form.