Go to your messages tab and click the conversation name on the left side of the page to make it active.

Next, click the options button at the top right and choose Edit conversation.

In the pop-up, you will see the current list of members.

To remove someone, just click the x next to their name. To add different members, you can select according to location, position, or individual names:

Save your changes and the conversation will be updated. 👍

Go to your messages tab and tap on the conversation you'd like to edit.

With the conversation active, tap the options button at the top right, then choose Edit conversation.

You'll see the conversation name and total number of members. Tap the employees row to add or remove people.

In the employee list, tap a name to check or uncheck members, then tap Save at the top. 

Confirm your changes on the next page by tapping Save again and your conversation will be updated. 👍