If you'd like to collaborate with external contractors, consultants, etc. you can invite guest users to your organization's Team. Guest Users have almost the same capabilities as regular Teams users (see details here).
Here's the official Microsoft guide on how to add guests to a team in Teams.
If you're a Teams Owner and you can't add guests to your Team, please contact the Help Desk and we can turn on Guest Access for you.
A guest user will have access to all the public Channels within the Team(s) they are assigned to. If there is information you don't want to share with a guest, we highly recommend creating a separate Team that guests can have access to.
For example, you could create a general "Guest Access" Team so staff who are members of that Team are aware that third parties have access to that information. From there, if you have multiple third parties working on multiple projects, but you don't want them to see each others' work, you can create Private Channels per project within your "Guest Access" Team, and assign Guest Users access to those Private Channels accordingly.
Always feel free to ask the Help Desk if you have any questions or hesitations and we'll get you on the right track!