You can invite Users at either the Organisation level AND/OR at the Register level.

Both settings can be found on the left hand side menu of the AssetAccountant application.

Which you choose for a particular User will depend entirely on what permissions you intend to allow for them.

This guide will assist you choose: 

User Roles in AssetAccountant

If you are inviting a new User at the higher Organisation level, you can allow them access to multiple registers at the same time you are inviting them:

Be mindful of permissions. If you have invited a new User as only an "Account Member" at the Organisation level, they will not automatically have access to nested Registers, unless you invite them to specific registers that you intend them to have access to.