I agree with Mandana for the most part. End user employees of the target company should be met with early on and explained the integration and why it matters to them. Periodically they should be provided updates on how things are going as well. Key employees in the target company on the other hand should be integrated into the integration team very quickly and included in the regular communication cadences – weekly meetings, report-outs, regular 1×1 check-ins, etc.. The end user employees will be important to keep up to date, while its critical the identified key employees are looped in quickly and repeatedly.