I currently have MS and have been treated for such under my individual health insurance plan ( I do not work and my husband is self-employed). If he gets a job working for a company and we switch our insurance plan to a group/employer covered plan, will my MS be considered a pre-existing condition and what do I need to be aware of before switching?

Yes, it would be considered a preexisting condition as you had this condition before getting their insurance. The best bet in your case would be to wait until the employer has an "open enrollment" period and have hubby get the insurancee then. In open enrollment periods pre-existing conditions are generally covered. Have him check this out before he signs up for any insurance with his new job.

Typically a new hire is a qualifying event. Meaning that once you meet the waiting period the employer has set for new hires you can enroll in the plan and not need to wait for the annual open enrollment.
As long as you have had uninterrupted creditable coverage of a similar type. A group could only exclude coverage for a pre existing condition for a period of 12 months after your enrollment date. And that will be offset by your current creditable coverage.
So you should have no waiting period for coverage as long as you keep what you have now until your spouse is on his new plan
Thase are Federal HIPAA regs some states do not allow Pre X at all.