Military members are special people who have dedicated their lives to their country. Properly addressing a wedding invitation to a military member will show your thankfulness for their service. The outer envelope of a wedding invitation going to a member of the military should be addressed using the full rank of the military member. Do not use the abbreviated title of their rank.
An example of a properly addressed envelope would be, “Major and Mrs. Jones” or “Sergeant First Class and Mrs. Smith.” If both man and wife are in the military and have the same rank you would use, “Captains Christopher and Mary Wise.” In this situation the inner envelope would be addressed, “The Captains Wise.” If both man and wife are military members but have different ranks you would address their situation like this, “Specialist Mark Allen and Staff Sergeant Mary Allen.” The inner envelopes will be addressed the same way but omitting the first names.
If knowing what to say is not the problem but finding the right invitation is then look here to see what design might fit you best.