Creating Groups - 

• Go to the SharePoint team site, details on getting here can be found in the article on accessing the SharePoint Admin Centre.
• Click on the settings cog in the top right corner and select site settings
• Click on the People and Groups link
• This will default you into the 'Team Site Members' group, click on 'Groups' above this.
• You will see all the default groups, click on New -> New Group
• Name the group and run through the other settings, such as who can edit the group, and the groups permission to the site.
• Click Create
• Now click New -> Add users to add the required people to the group

Giving Access - 

• Now you have a group you can give that group access to certain folders on the site.
• To do this go back to Documents, select the folder that you want to give them permission to and then select 'Share'
• In the invite people you can give basic permissions, its better to click on 'Shared With' and then 'Advanced' at the bottom.
• By default the folder will inherit its permissions from the one above, click the diable inheritance to set unique permission option
• Now remove any groups / users that shouldn't have access by clicking the tick box next to them and then the 'Remove User permissions' button
• Add the required groups by clicking 'Grant Permissions' 
• In the window enter the group name and then click show options at the bottom, this will then let you select from all of the permission levels
• Click Share.

• Click on the Settings cog and select site settings
• Click on Site Permissions
• On the Permissions tab at the top there is a 'Check Permissions' button, click this
• Enter the name of a group or user and it will give you a list of the groups they are in and the permission level
• To check the permissions for a particular folder go to the folder and click on share
• Then go to shared with, this will show you who has permission but not what level
• So click on Advanced
• This will tell you which groups have access and what permission level
• You can click check permissions at the top and enter a users name to see what access they have rather than having to check through all the groups.