I used Galaxy quite a bit during my PhD project, using the online resources instead of starting my own personal server due to better resources. Thus on my galaxy account I have filled up 90+% of my space and my advisor is still using this information. However, my new postdoc also could make use of a Galaxy account I wanted to start a fresh one for these purposes. Would that be OK to do, or do I have to try to compartmentalize and manage with only 10% of the space from my old account until my advisor is done using this information?
As long as the new account is registered under a distinct email address and is used only by that person, this should be fine.
Another choice is to reassign the existing account's owner to your advisor. The account email address can be modified under "User > Preferences > Manage your information". The password can be left the same or changed. Then, create a new account for yourself and only use that new account going forward.
Note: If you do the second, your advisor will need to "activate" the account within six hours, so coordinate appropriately. They will receive an activation confirmation email at the new registered email address. The process is very simple, just click on the link in the email (or paste it into a browser window).
Sharing the data with your advisor's account (if they already have one) will not solve the problem of your account being nearly full.
Any particular user using multiple accounts runs the risk of being detected and the account being deleted. This is something you certainly want to avoid as data is sometimes completely removed upon deletion.
Hope one of these options works out for you! Jen, Galaxy account.