What you would like to take first is Empowering Charge, because as a melee support, you help other melees (or Valla if no warrior in team).

Feral Heart gives you health and mana regeneration while in wolf form. You should only be in wolf form for a long time when you travel to other party members, because wolf form is very good to use as a poke, last kill and escape mechanic (more on that later).

Then you take Shadow Wolf that cloaks you from enemies for 2 seconds. I use this to get away from death, and it has saved me almost 90% of the time I have played Rehgar. Usually there is some or someone chasing you and others the ally. If you are with Valla, you can both escape. Just hand her a heal if she is really low on health so that she can't be sniped or something. Activating wolf form again takes about 5 seconds. So be sure you know when to use it and when not to.

Bloodlust is an incredible buff for you and allies. You can use it not only to fight an enemy player group, but also for taking out towers with your allies (even not being there, more on that later). This is also a good chase mechanic, so you can use it after the team fight has gone on for awhile, and then chase the heroes down. Use it later if you are certain that your team can win the fight. If you are lower level, use it immediately in a team fight.

Last but not least, go for Gladiator's War Shout. It makes your Bloodlust give all allied heroes, minions and mercenaries a buff, GLOBALLY. Meaning you can be at your spawn place (and your allies fighting towers or enemy players) and give them all a buff without being near them.

Healing Surge makes you not only a buffer and chaser/escape artist (with area slow, more on that later). This makes your healing bounce on one more friend and heal the primary target by 25% more. Excellent for team fights, but also solo or 2v2.

The best way to use Earthgrasp Totem passive is when you escape or chase. Also use it to root enemy players in team fights. If they are all in the middle, it's a lot of valueable time lost for them. This also increases the cast range of your area slow by 100%, making all the things before-mentioned way easier.