Start by recalling the laws of energetic: you get back what you put in, everything. People will act toward you, just the way you're acting toward them.
Be the builder, the giver, not just the reciever. Focus on increasing the bond in your relationship by giving space, having useful reason to hang out, being sentimental and empathic, etc. And it does not mean are going to let yourseld be taken advantage of, be used nor be manipulated in any way.
Invite them to go along, if you want to be invited. Make them feel some importance and you're going to be of a lot importance.
Cheer up, smile, be a fun person to be around with. Teach, create, tease. All of this makes you stand out as a significant individual, creates your personality, and make people regard you as an important social asset--irresistable to be left behind.
Appreciate your friends. Show them how glad you are to have them. Use moments. Occationally. Less formally. But genuinely.
Be realistic that you don't have to be everywhere in their lives. Have time for yourself and allow them to do the same.
Stop overreacting or creating a negative scene when you're invited. Do not seek for attention or make the outine your own.
When you're not invited on an outing, be eager to know how it goes, with a deep and positive interest, instead of showing attitude.
Never allow yourself be used as an ATM machine just because you're invited on an outine. Respect your personality and let wisdon work you out of those kinds of companies.
Only a healthy relationship can save you from manipative friends. A bad company is a bad company. And you cannot change it. Make sure you're in a good relationship.