Workspaces contain collections, user groups and users.
Workspaces are a way for your account to be in multiple contexts, for example if you might have multiple companies. They are not a way for you to group users in 1 company.
User groups are a way to group users in 1 company, e.g. your departments. User groups allow you to assign access policies to certain collections so that only some users are able to see or edit content.
This will work against you. Search works at workspace level, thus you will not be able to search for information in all your workspaces. It's also not possible to mention people & docs from other workspaces as mentions and docs are at the workspace level as well.
Collections are a way to group documents. They are similar to projects in your company. They can be assigned access policies so that only certain groups of people.
Users are members of 1 company.