Forum Discussion
SharePoint Sites For External Users
For my customers, I usually create a Group for each of their clients.
In this way, apart from the site collection (with its team site and document library), you also have automatically a conversation space, a calendar and a notebook, which could be very useful.
Subsequently, you can add Planner and Teams, if needed.
This is the trend today...
Don't mix clients in the same site collection/office group, this can quickly lead to embarassing/legal problems.
SPO has the Site Policies which allow you close down projects/sites, these can easily be leveraged with client specific site collections.
If you have projects with many firms involved, then you can have separate document libraries as needed to separate the sensitive content, but this will get tricky if you are using Office Groups. You may want to create a permissions matrix to make sure it gets implemented to meet your requirements