You can create a new group chat on Instagram by sending a message to 2 or more people. To create a new group chat:

Instagram app for Android and iPhone
Once you create a group conversation, you can add a name for it. To name a group conversation, tap the group conversation and then tap Name this group... at the top.
Group chats with Facebook accounts
If you’ve upgraded to
Messenger features on Instagram, you can create a new group chat with Instagram and Facebook accounts. Note that you can’t add Facebook accounts to existing group chats that only include Instagram accounts.
Some features aren’t available for group chats that include Facebook accounts like:
- Message forwarding and vanish mode
- Animated selfie stickers
- Sending and receiving payments
- Restricting a member of the group
- Ignoring or hiding the group
- Marking messages as unread