Similar to above you could try a couple of things -

First , set a set % available for the resource time. For example if you have access to timesheets then you should try and come up with historical data and use that as basis for resource allocation for new projects.

So, if you find out the resources spend 30% on average then the resource availability of all the resources should be 70%.

Second, make it clear in the project initiation or business case or any document you use to initiate a project that the resources are not FULL TIME.

Hope it helps.

Similar to above you could try a couple of things -

First , set a set % available for the resource time. For example if you have access to timesheets then you should try and come up with historical data and use that as basis for resource allocation for new projects.

So, if you find out the resources spend 30% on average on other tasks then the resource availability of all the resources should be 70%.

Second, make it clear in the project initiation or business case or any document you use to initiate a project that the resources are not FULL TIME.

Hope it helps.