There has been a LOT of discussion lately on the whole "adding a person" concept. Most of the people here will very adamantly tell you that it doesn't work. Your relationship with your husband is just that. YOUR relationship. The relationship with someone else will be your relationship with someone else, and your husband's relationship with that person will be his relationship with that person.
is a thread specifically geared towards the "adding to/joining" mindset and why it generally doesn't work.
However, a poly relationship CAN work. Both triads (all three of you involved with one another) and vees (one person being involved with two others while the two "arms" are not romantically involved but often friends) can be successful as long as everyone has similar expectations and honest and open communication are kept up.
Read around; there's tons of information about various relationship models, hierarchy of relationships vs equality (what you seem to want is a hierarchical model where your relationship with your husband is and always will be primary meaning that any additional partners will by default come second - there are people here that will argue for and against this concept), finding single people who are interested, how people go about dating, etc. There is a wealth of knowledge!