Virtua Rasa users can have one of three roles in a world (four if you count no access at all). They are:
Gamemaster: Gamemasters have full read/write access and can add/remove users, create/edit units and maps, and edit the world's wiki. They can chat as any unit or other name they wish, although their default chat is the same as a Player's.
Player: Players have limited read/write access. They can view the world and edit/move units a Gamemaster has given them access to. They have edit permission to the wiki. They chat as either their username, or their default unit if they have been assigned one.
Watcher: Watchers are strictly read only. They can see map changes and chat messages, and read the wiki, but cannot move units, chat, or edit the wiki.
To grant a user access to a world, or to change their existing access, follow 'Profile->All Users' to go to the user management page. There will be some information about how many users of each role the world supports, and a table listing the current users in the world.
Important: as you make changes, the rows in the table will get a yellow background and be changed to italics. That indicates the changes are pending but have not been submitted yet. Click the 'Update' button to submit the changes. Click the 'Cancel' button to drop the changes.
To add a player you will need to know their username. Enter their username in the 'Add Username' text field, select the desired role, and click the 'Add' button. This will add a row to the table as a pending change (see above). To change a user's role, select the new role from the menu next to their name. If you assign them a role of 'None', they will be removed from the world. Remember to click 'Update' to confirm your changes.
Players may have units assigned to them in two ways:
Default Unit: A player may have one default unit, which will be used to determine the name and colors they interact with in the map. If a new default unit is assigned, the previous one will be overwritten. Multiple players may have the same default unit, but that will most likely be rather confusing. A player cannot move or change their default unit unless they are also granted edit access.
Edit Access: If a player has edit access to a unit, they have full read/write access to the unit. They can change name, colors, icons, etc. Importantly, they can move the unit on the map. A player may edit access to multiple units, and multiple players may have edit access to the same unit.
To change unit assignments, follow Profile->Manage->Units to get to the manage units page. Select the unit you desire to change. (There is an interactive search field that may be useful if you have a lot of units). At the bottom will be a table of the users in the world along with checkboxes indicating current assignments. Change the checkboxes as you desire, and click 'Update'.