Things may look a little different in UpContent as of 09/14/2022, check out the new update here.
Begin by accessing the team settings screen by clicking on the account name, and then "Team Members".
Once you have accessed the Team Members screen (which you can access directly by going to https://my.upcontent.com/team-members) you can assign a team member by finding the appropriate one and clicking "edit" under the "Assignments" header.
From here, select from the drop-down menu which folder you would like to assign them to, as well as the permission level you would like them to have before clicking "Assign Folder" to finish the process.
Repeat the process to assign multiple folders to the same team member(s).