If you are unable to share with a specific user, it may be because that user is on a different payment tier than you are. For example, users on the Pro tier can't share projects with users on the Free or Basic tiers.
The reason for this is because our pricing is based on connectivity. If your team needs to connect to Snowflake, which is only available on the Pro tier, then you could just get one Pro license, load data with that account, and then share it with the other accounts.
In order to keep pricing as low as possible for everyone (including people on the free tier), we must have this limitation in place.