Hi @majorlegendx 
Welcome to the Forum!
Definitely not a stupid question, it sounds like you’re really close. If you have a Data Type called “Chemical”, you can create a new field under it called “Users” (type: User, List: Yes). Then use the Make Changes to a Thing, Thing to Change: Chemical, Field to Change: Users add Current User (or whichever User is relevant here).
The error message in the repeating group is pulled up because you need to select a text field, right after selecting the User Data Type. For example, the dynamic expression could be “Current cell User’s email”. Email is a text field, so that would display correctly. Alternatively, you could create other fields such as “First Name” (Type: Text, List: No), “Biography” (Type: Text, List: No), etc.
Are you trying to display the Chemicals, with the list of Users associated with each Chemical in a repeating group? If so, you would set the type of content of the RG to “Chemical”, Data Source: Do A Search for Chemicals. Then (there are a few ways to do this) but you can have the Users field (Type: Users) within the Chemical data type, your text element in the Chemical repeating group to show those Users could be “Current cell’s Chemical’s User’s First Name”.
However, this isn’t a very flexible solution since all of the Users first names will just display in a long paragraph separated by commas. Instead, you can place a second repeating group inside the first cell of the Chemical RG. This would be a nested repeating group which displayed Users associated with the Chemical. The Type of Content would be User, and the data source would be: “Current cell’s Chemical’s Users”. Then place a text element within the first cell of the User RG, with a text element such as “Current cell’s User’s first name Current cell’s User’s last name”. This way, you can easily display the full names (and any other dynamic data, such as a profile picture) associated with each User who likes that Chemical.
This is probably a lot of info at once… If any of this is unclear, feel free to share a link to your app in the editor (set to public) and we can definitely help you get this set up!