Rosterr identifies a workspace by your Roblox group ID. It is a number.
Find it
Open your group's page on the Roblox website.
Look at the web address. It contains the number, for example
roblox.com/communities/12345/.... 12345 is the group ID.Copy just the number.
You can also paste the whole group link when Rosterr asks for the group ID, and it picks out the number.
It is also used as the workspace ID
In the game API, the workspace ID is the same number: your Roblox group ID.
If Rosterr says the group is not valid
Check you copied the number from your own group, not a game or a user.
Make sure the group still exists.
A group that already has a workspace cannot be used again.
And you need a rank of 25 or higher
To create a workspace for a group you must hold a high enough rank in it. See "How do I create a workspace for my Roblox group?".
Related articles
How do I create a workspace for my Roblox group?
How do I get access to Nebula Rosterr? (waitlist and early access)