Learn Finnish: Turistit tutkivat museota ryhmittäin. - The tourists explore the museum in groups.

It uses the distributive suffix -ittäin, which translates to 'in' or 'by' (as in 'in groups' or 'group by group'). To attach it, you must use the plural stem of the noun. For ryhmä, the plural stem is ryhmi-. Adding the suffix gives us ryhmittäin.

Why is the suffix spelled -ittäin with ä's instead of -ittain with a's?

This is due to vowel harmony. The root word ryhmä contains the front vowels y and ä. Because of this, the suffix must also use front vowels, making it -ittäin.

Could I just say ryhmissä (in groups) instead of ryhmittäin?

You could, and people would understand you just fine! However, ryhmittäin specifically emphasizes the manner of organization. It highlights that the tourists were sorted or processed group by group, rather than just pointing out their physical location inside groups.

Why is the object museota in the partitive case?

The verb tutkia (to explore or examine) frequently takes a partitive object because the action is ongoing and open-ended. The tourists are spending time exploring the museum, rather than completing a final, finite action to it.