In order to give someone access or remove someone's access in a particular project you need to first make sure that those people are part of your team.
Note: Only the project owner or a project adminstrator can give or remove access to a project.
It is very easy to find out who has access to your project.
Access button blue = Person has access to selected project
Access button gray = Person doesn't have access to the selected project
Give someone access to a project
Click on your avatar in the project.
Select the user you would like to give access to in this particular project and then click on the Access button
Once you have clicked on the Access button it switches from gray to blue. This means that this person now has access to this particual project.
Once you have given them access, the project will now be visible for them in the menu like shown in the screenshot below.
How to remove someone from a project
Follow the steps above, however simply click on Access button again so it switches from blue to gray.
Once it is switched back to gray, that person will not have access to your project anymore.